I used a couple 4'x8' sheets of 2 inch thick pink housing insulation foam from the Home Depot aviation supply store.  I used a metal straightedge and scored halfway thru with a razor knife and then snapped it. Quick, easy and not too expensive.

At some point in the past I did an experiment in which I put a block of this foam on a scale and pushed down on it with the end of a 1/2 inch rod.  I remember the rod begining to penetrate at about 50-100 PSI.  The load pressure supporting the plane is well below this, perhaps 5 PSI max.

I thought about making a concave top piece of foam, contoured to fit the wing. But, I tried it first without that, and found it worked quite well to just use a flat top piece.