You are going overboard on this issue. Anything that would withstand 200 F max would be more than adequate. In other words, any sort of canvas type material that is used for convertible tops on boats or cars should do the trick . After all, these engine covers do not sit directly on the exhaust system. Check your local Yellow Page directory for companies in the "pop top" business and talk with them. Take them a sample of your old cover. These guys make all kinds of stuff for people. Even the material used to make covers for BBQ's should be fine. There are also materials that are flame-retardant. Yes they will burn but if the open flame is removed, they will self-extinguish.