Hi, and thanks for the interest! I'm really happy to see the helpful positive help coming in at last. ;=]Do I understand what you are thinking?
The PVC pipes are to connect the edges of the foam together? Won't you still have the problem of the loads going through the foam? The foam itself is acting as the spar?
Why not post a picture? Just a quick sketch would do. Better yet, a photo.
I must Draw my SkySled concept and glue some prototype pieces together to show y'all puzackly what I think.
But, in brief, think of a reversed Facetmobile where the fabric cover is replaced by a Styrofoam XPS brand air frame/fuselage and all of the interior trussed aluminum tube stress-distribution network is GONE, replaced by a few Styrofoam shear partitions. That's monocogue . Hopefully cheeper, easier and stronger and lighter, as Burt Rutan claims. [I bought his book] We'll see.