Eddie,
The obvious answer is to use 1/20 balsa! I did a search and found these guys who sell 1/20 sheets:
http://www.specializedbalsa.com/prod...lsa_sheets.php
The free flight guys use 1/20 stuff. Honestly, I would just use 1/16 Sig Contest Balsa and sand it really smooth after its glued on the foam. You may be surprised at how much you take off with just sanding to get a smooth hull line.
Tailwinds,
John