At most home improvement chains you can get foil coated foam board in full sheets. The foiled (not in the Snidley Whiplash sense though) side should be eco(nomically as in polyester) FG resin friendly. The foil layer also provides radiant shielding so your truck bed under such a foam-glassed cap wouldn't become a solar sauna.
BTW, I've found TAPPlastics.com a good place to buy resin and hardner. Anyone have other recommended sources?