.005" thick won't do the job of supporting the carb, IMO. Use 3/32 - 1/8 thick angle stock. Don't worry about the differential coefficient of thermal expansion between the carb, head, and the crankcase. From my measurements, the head will run around 300F (maybe a bit more right at the exhaust mani). The case will top out at around 150 degs F. So, aluminum's Tce is around 13.1 u"/inch/degF. This means that your engine's head will grow around .006" max. The bracket, since it's cantilevered, will easily flex this much. No worry.

As for the plastic, one cheap way to get better performance is to go to the grocery store and buy a small white teflon hot plate. They're cheap, and have very high temperature tolerance. Another material is cotton or glass impregnated phenolic. These are available at hot rod store or on Ebay as carb spacers for race cars. Kind of expensive. Also, you can buy phenolic stock right on Ebay much cheaper. Personally, I'd try the hot plate material teflon. Good luck.