I too have used plastic. Cutting boards look promising. My stash is unknown plastic of various thicknesses. None has melted or given me any trouble. Perhaps a plastics shop would sell or give you some scraps? I have also used aluminum with only thick gaskets to act as insulators. That worked too and did not cause the gas in the carb to boil. Mostly I have been using 1/4" phenolic board lately. It has enough thickness and strength to give the carb screws a good bite. I have not noticed any problems in how it runs with the short intake plenum. Maybe try 1/4 or 3/8" aluminum with thick gaskets?