For maximum rigidity in a given volume then I think that’s correct. I think it depends a lot on how forces are able to be transferred through the ribs, though. A set of well fitted ribs should still give significant torsional stiffness without a bottom skin, however adding a bottom skin will always improve rigidity.
I guess that’s to say that it may well be plenty stiff enough with just a top skin, or that if a bottom skin isn’t desirable then taller ribs could still work. I don’t think it ‘needs’ the bottom skin inherently.
I was having a play with a design in the Solidworks mechanical simulation addon but I’ve since lost access to that license, sadly.
First side is skinned with 1/4” underlayment. It’s mostly flat, not perfect but close enough. Once it’s dry (and after the Chiefs win in a couple hours), I’ll flip it over, shim it as flat as I can and put the second skin on.
I had very tight fitting but perfectly tight fitting joints, and I did use glue which act as lubrication believe it or not and then a clamp that press the two together, this was after I used the hammer and broke some ribs lol I use glue on the joints on the ribs and I also use glue on the quarter inch sheeting top and bottom and used weight And clamps around the edges weight in the field and no nails or staples anywhere
Also to keep it lightweight so I can move it by myself full sheet I used half inch plywood for my ribs and struts
