RE: This is gonna be tough...
Either some anhedral and an upper plastic fin or some dihedral and a lower fin. One thing with the clear plastic is that it could be extended back behind the elevator so it can stay as small as possible.
The ideal material would be some lexan sheet. You could get away with a thin piece since it's so tough. Most plastics shops that deal with it should be able to sell you a scrap the size you need for a buck or two. Something in the .050 to .060 range would be aces.