Ok, I've managed to grab a few minutes on sketchup and here is pic of the torsion box.

The method I've used makes for an extremely rigid construction. The top and bottom would be 15mm alu, and the inside of the box would be 10mm alu with the perimeter double skinned to allow plenty for any fastenings.
Click image for larger version. 

Name:	torsion box open.bmp 
Views:	464 
Size:	2.93 MB 
ID:	10320

Ps, the overall size of the box is 1100mm x 700mm x 130mm