Thanks to all for the tips. I found the perfect material at a signmaking
store, it is called Coroplast:
http://www.coroplast.com/general.htm

Two layers of this stuff is exactly the height of the floor corrugations. I
cut strips at right angles to the grain of the Coroplast and it is great.
In fact, a sheet when place perpendicular to the corrugations seems rigid
enough to support walking on. I then put the 1/2 inch thick Rubatex Open
cell foam on it
http://www.rbxcorp.com/
with hemp canvas on top (That's right Mike, I did say HEMP!)
What a great floor!

-- Rachel
http://www.Gnarlodious.com/Vanagon/Vanagon.html