Ditto on the winter beating down on the mileage. It happens every year and for whatever reason, I always think I can get the same MPG's as summer... or at least, not such a harsh hit.
Fiberglassing... huge pain. I'd say layer it!
Maybe take a look at the way I did mine:
http://ecomodder.com/forum/showthrea...tml#post516730
You have the shape already. Coroplast, a bit of scrap steel or aluminum, some spare retainer clips, and duct tape or paint later... I have mine a little on the long side, so when you close it, it forces the coroplast to angle a bit against the bumper, creating a decent seal without needing an additional part.