I know the pink stuff very well, used many sheets on my train layout for scenery the new purple has better R factor, as for rain, Calgary is almost a desert re: rainfall, and the snow is dry ;^)
I did not know about the purple stuff ( I better check it out at Ronna or Home Depot )
I did know about Calgary being a desert, in Toronto, is humid and a lot of wet snows.
What about the electronics "boxs" ( ABS, cruse control, engine control , ect ) inside your car ( specially made for Canada ), maybe a similar construction technique will be usefull for your project.
Just a taught...