I once bought a $10 trash can to make an air dam extender (stole the idea from Jay). it was a basic gray. you could buy one and cut material out of that to put there and it would look descent. you could also spray paint it with Krylon Fusion paint that is made for plastic.
you could cut a piece way too wide and use a dremel to get the right angles and fit it in pretty easy.
the same could be done with plexi-glass but that does get expensive.
Be the change you wish to see in the world
Actually, I have some sheets of plexi-glas that are scratched and stained, they'd be good with some paint (or a sheet of rubber laminated on) and would not end up wavy like every other idea I've considered. That's the best idea yet.