I was going to post in the other thread but you started this one five minutes later.
If you like (itchy, sticky) fiberglass then 0kay, but personally I'd go with Polymetal [or equivalent] or 1/8" sheet ABS. Polymetal is stiffer, but ABS can be heat-formed and cemented.
A Bonneville spoiler is subjected to low pressure on the bottom, just as a diverter is subjected to high pressure on the top. Use turnbuckles with magnets on the end.
