If you are like most RC enthusiasts you have spare spare motors, servos & batteries. This instructable shows you how to make a flat foam airframe in a couple hours.
I've modified free plans from http://www.foamyfactory.com/
for a faster build that is also stronger.
This requires just 1 foam core board that can be purchased from a craft store.
I used the 1/2" thick board instead of the more common 3/16" version I've used in the past.