When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
When I lowered my car, I cut off the rubber spacers and epoxied a piece of cutting board in it's place. I used a piece 3/8" thick because there is a 1/4" depression in the frame above the spring spacers and I didn't want the spring resting on the frame.
I wouldn't feel comfortable glueing 2 pieces together, so It would be best to buy material in the thickness you need. Any polyurethane or similar material should be fine. It should be soft enough so it doesn't damage the spring.
Thanks to, I hope I recall correctly, member Agent86 for the cutting board idea.
Oh, just had another idea. The stock setup uses 1/4" metal spacers with holes for the bolts to run through. You might be able to get some from people who are lowering their cars. I no longer have mine, or I'd offer to send them to you.