Being limited in experience with RC, I can't say how they'd perform long-term on a plane engine. However, I've used baseball card thickness cardboard home-made gaskets on cars and small engines since I was a kid, and it works fine.

Lay the cardboard on the contact surface. Take the handle of a screwdriver and tap the top of the cardboard all around the edge of the contact surface to mark the shape you want. Use scissors or a hobby knife to cut it out. Coat with gasket sealant and install. I've actually got a home-made muffler gasket on my OS 46 on my cub that I made this way. It has a good bit of run time on it without showing signs of burning away. Although, like someone else mentioned, I have no idea why you would need a gasket here in the first place. I just put one back on because one was there before.