I'd say u are in a questionable temp zone there. Considering it is alot of hours going into a project, use polycarbonate (lexan), which holds up to 120C. Which will allow you to use a slight overwatt H9 bulb (if it ever arrives on the marked).
I checked
http://www.tayloredindustries.com/electricalproducts/arboron.htm
and it shows 0.5 - 0.9% water absorbtion, which did not look good to me. The applications listed for this material is much drier than headlamp bracket. I assume you would like to drive your car in rain? So that could be a big question mark.