I believe the RS rule for this is based on GT regs which allow any clear uncolored polycarbonate material as long as it is .125" thick or more. This would include trypical lexan and FAL's "flexan" derivative which is more scratch resistant and won't discolor from heat and UV exposure. I know their race kits are .125", but double check on the formed kit...its what you will want as its just the 3 rear windows and is preformed for factory style fit.

-dw