Hi all.
I wanted to share my experience with my hand-made diff mount on my C5 Z06 if anybody interested.
I made it in effort to combat the wheel hop I had with my Z06. Fast-forward it did not fixed the wheel hop, so it needs something else.
But the mount after 1000 miles turned out to perform well, and it is much better IMO than the stock one. Once I installed it I did not notice any difference from the stock mount vibration/noise-wise. So I believe it is safe to install.
So the whole point of this post is to try to share this mount build process. If someone is willing to repeat my steps i must warn that you are doing it at your own risk, no guarantees it will work the same as mine.

So let's start.
First of all here is the list of tools you will need:

• Drill press - although not strictly necessary is highly recommended, or you can use hand drill of your choice;
• Metal saw - any kind to cut aluminum and steel;
• Thread taps - I used M4s to make nut stoppers, you can use any reasonable size, or if you have welder you can just weld in the nuts;
• Wrenches;
• Sand paper;
• Primer and paint that works on metals.
• Some small stuff - cleaning solutions, rugs, tape, plastic cards, drill bits, etc.

Now the list of materials you need:

1. Liquid polyurethane - something like this: https://www.energysuspensionparts.com/diymmi, I used 80a hardness, but now I believe 60a will be even better,
2. Universal bump stops - something like https://www.energysuspensionparts.com/9.9118,
3. Steel square tube - 1-1/4 X 1-1/4 X 3/16 wall A500 Square Steel Tube (Custom Cut: 1' 6"),
4. Inner aluminum channel - 1-1/4 X 1-1/4 X 1/8 6063-T52 Aluminum Channel (Sharp Corner), any aluminum alloy should work.
5. Outer aluminum channel - 2" (2.38" OD x 0.22" Wall x 1.939" ID) Schedule 80 6061-T6 Extruded Aluminum Pipe, any aluminum alloy should work.
6. Bolts to attach the diff - 2 x M10-1.5 x 60 MM Metric Allen Socket Head Cap Screws Grade/Class 12.9, like those:
   https://www.amazon.com/gp/product/B078SDJJN3/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
   https://www.amazon.com/gp/product/B078SDJJN3/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
7. Bolts to attach the mount to subframe - 2 x M10-1.5 x 35 MM hex bolt, you can reuse the stock mount nuts, or buy new nuts,
8. Bolts to attach the mount to square pipe - 2 x M10-1.5 x 45 MM hex bolt, you can reuse the stock mount nuts, or buy new nuts,
9. Cross mount bolts - 2 x M8 or any close SAE size bolt and nuts, length 65-70 mm or similar (they just need to stick out from the mount well enough).
10. Bolt twist stoppers - 4 x M4 8-10 MM set screws.
11. End caps - 1 inch square:
    https://www.amazon.com/gp/product/B01NBI194S/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
    https://www.amazon.com/gp/product/B01NBI194S/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
    , to close squeare tube ends.
12. M10 washers (narrow) - 4
13. M8 washers for bump stops - test fit the final product, add if necessary

Ok so the mount itself was inspired by the well-known PFADT series transmission mount. But the central bushing is close in construction to a stock diff mount instead of the round one from PFADT mount. It consists of a horizontal support bar in a form of steel thick-wall square tube (3), that has polyurethane bump stops at the ends (2), end square end caps, closing open ends (11). This bar is bolted with bolts (6) to the diff, and bolted with bolts (8) to the aluminum-polyurethane mount. The mount itself consists of two halves (4) and (5) that are thick walls aluminum square channels. inner channel is inserted into the outer channel, and supported by the polyurethane (1) in between them, and two cross bolts (9), those bolts are the safety measure, in case the mount is split apart, and something is still holding it together. Bolts (7) and (8) are screwed into the channels (5) and (4) and are fixed from twisting by set screws (10) and by the polyurethane mass (1) around them (although I am not sure that this is a reliable mechanism, but it worked for me). Finally the whole assembly is bolted to the subframe by bolts (7).

This is my poorly hand-drawn plan for the main parts. All measurements are in MMs, and might not be entirely precise, since some parts I cut to size myself and they turned out to be not 100% accurate.



Here are some build process steps with pictures:

• Preparing square tube. Sanding the surface. Putting dimension on it. Since it was pre cut to size (1 ft 6 inches) no need to cut.
• Drilling holes in steel square tube with my drill press. Dimensions are from the plans above. Image looks distorted because I shot it with panorama setting.
• Drilling holes in inner/outer square channels with my drill press. Tapping M10 threads. Dimensions are from the plans above. Outer channel. Note: Twist stopper holes positions for set screws are arbitrary. They just need to be in touch with M10 bolt hex caps, to prevent twist. To easily get the right positions, just screw in the M10 bolts in place, tighten, then mark the appropriate set screw hole places. The should touch hex cap edges. Outer channel. 8MM holes for M8 cross safety bolts Inner channel. Same technique for set screw holes as for "outer channel" Inner channel. Note: The holes here are 12MM to allow inner channel travel inside outer channel when they are tied by polyurethane. the hole should be considerably bigger than M8 bolt. I think 2MM gap is enough. Also note the additional pair of holes. those are to increase polyurethane bonding to the aluminum part. The number and size of holes could be any, just do not compromise the structural strength of the part.
• Inserting M10 bolts into inner/outer parts. M10 bolts, M4 set screws Here the set screw locations are visible Here it is clear how set screws are grabbing to the bolts hex caps.
• Preparing the elastic mount assembly from outer/inner parts and cross bolts. preparations are needed to pour liquid polyurethane into the part to bind everything together. I used some plastic cards to hang the inner part on the right level, so the cross bolts are centered inside oversized holes in the inner part. Also centering the inner part within the outer part. This will enable polyurethane to fill the hole voids evenly, allowing the mount to flex uniformly. I again used so plastic cards scraps and tape to create a sealed mold, so the polyurethane will stay in place.
• Now it is time to pour some polyurethane. The parts are centered (not ideally, but good enough). I deliberately left gap on left side bigger to make it easier to pour in the polyurethane. I created a funnel with another plastic scrap piece, and poured the polyurethane in.
• Ok it is now time to assemble everything. First I need to paint steel tube, to prevent rust. I sprayed inside as well. Final result
• Now you basically need to screw everything together. I do not have detailed pictures on how to attach bump stops (2), but they are as simple as just bolting the nut on the stud. Final product. Note: it is not shown but, plastic caps are inserted into the open ends of the square tube. Also make sure to test fit the final product with your rear sub frame. I ended up adding few washers between the bump stop and the square tube, so the bump stops rest on the subframe. I decided not to put set screw twist stops for the bump stops studs. Since it turned out to be easy enough to just use a spanner and hold the nut while screwing the bump stop stud. So those M4 holes are not needed.
• And now how the fit of the mount on a car looks like.

That's about it. It works well, nothing is bent, the polyurethane is not torn. I do occasional burnouts (have old tires), so I still have wheel hop a bit, but the mount is solid so far.
Cost wise, it was well below $200 (do not remember exact number, I did it in 2018), and could be even lower if you have scrap metal, bolts, paint etc lying around.

 

Cool... I like when guys make their own parts!

It looks to me like the bolts stick out pretty far in the vertical direction. Did you have to drop the cross member off the nuts to install this? Or are you able to drop it onto the nuts w/2-3 threads left?

 

That's fairly similar to the one i build for my car, if anything yours is a bit beefier. I've got about 7500 miles on it and is was still in great shape when i tore the rear end out again this past winter.

 

I was actually doing this when I have my trans out, so I did not try to put it in without unbolting subframe. But I am pretty sure you need to unbolt it to install the mount.

 

I looked at making my own (I have a full machine shop in my garage), but considering the cost of material and time that would be spent, it wasn't worth it.

I bought this, installed it on a Saturday morning and moved on:
C5 CORVETTE HPI TRANSMISSION BRACE - $223.36

 

Probably need better shocks.

 

I installed C6 Z06 shocks 1000 miles ago.