we'll just have to agree to differ John ....
thanks for asking - the tablets keep most of whatever it is I have at bay, better than a year ago.
As some sort of dubious reference, our double bed is nothing special 20mm fibreglass foam sandwich supported only along the edges. I have a sneaking suspicion that fibreglass sandwich floor would span 2 - 3m without steel support (the mounts don't have to be at the ends of a 4m box. Our 4mx2mx50mm foam sandwich roof seems to survive ok.
I also struggle with why "most" sub-frames have steel along the long edges, I guess it depends if the sides of the box are structural.
But mostly, it really does depend on what's on top of the sub-frame as well as how the sub-frame is attached to the chassis.
I guess my picture is that we are somewhere in a transition like that in which cars with chassis became monocoque. Perhaps a bold question for this forum, but really "why do we need a sub-frame?". And hopefully the answer isn't "so we can attach tanks".
Sorry Brij, looking good so far!