A word of caution about autopart gasket material: I picked up some 1/32" thick rubber/ silicone material from Napa which was the thinnest gauge that they carried, but was about 3-4 times thicker than what the engine mfr. supplied. Thought that thicker would be better, WRONG ! I cinched the muffler down well, used threadlocker and a lockwasher, but after a few flights the gasket had compressed, the bolts came loose, lost my pressure, and a very expensive aircraft.

I've been using plastic playing cards for a number of years with no problems.

Hope this helps
Joe