Valve covers are made from carbon fiber mainly for one reason - cosmetics. It looks neat. Your best bet is vinyl ester resin. It is transparent and cheap. Epoxies will work but most are somewhat yellow and they almost all yellow with age. The pthalonitrile resin referenced above is not clear is very, very expensive and gets some of its high temp. performance from the microcracks that allow volatile presure in the resin to vent rather than blow it apart.