That is the exact problem I spoke of above before folks said "the cost"
My last post was to simply address the cost issue as a non-starter when you made your own.
The "nothing to screw into" is also easily solved by placing wood blocks inside the sandwich when making the panel, switch to a nailess adhesive, or use anchor bolts when needed. Gaps can be filled later with "Great Stuff" or something similar.
By the wa there is a great guy, Jamie, down in El Paso who uses SIPs for his sides and tops. http://www.happitrails.com/
I visited him when I drove across country in November.
BTW If you use epoxy gleue you do not need that much weight to clamp, although it will weaken the structural component a good deal.