The 33" gasket is what they give you to line the cutout. I didn't bother to use it myself. I am using the spacer. It was easy to install & I figured, what the heck ,I've got it I might as well use it.
Don't worry so much about the size of the opening as long as the unit fits and is not hitting on any side. When your done you can use tape to seal up the unit so debris doesn't become an issue. You can also make an intake shroud cover using sheets of black plastic that you can buy at any plastic/foam outlet. Or you can buy one from one of the forum venders.