You could build a foamy for the B40, but it wouldn't fly as well as a built up plane.
I've built a couple of 48" span foamies for glow power. This is with 1/2" thick Foamular pink color high density insulation from the local hardware store.

If you used anything thiner than that, such as the popular 6mm Depron you would have to do a lot of reinforcing and probably also have to use "flying wires" attached between a pylon and the wing for enough strength.
The flat plate airfoil does ok on the small lightweight planes, but IMHO with a bigger heavier plane it just doesn't work out for electric flight. You need a good airfoil that will allow the plane to fly slower.

Bill