DBC, first ,you WIL NOT BENd 3/4 ply !

I assume you are talking about the roof, that is usually made with 1/8 inch ply or paneling Then the finishe skin on top of that. Some find 1/8 ply hard to find . You may want to start looking for it now.
I did not insulate the floor , I installled the floor mats that you use in a shop, the kind that looks like a puzzle & the pices are 2' x2' made of dense foam rubber. It is nice to sit on if I ever take my mattress out, also some consider the mattress to be plenty of insulation . I did not see a need to insulate the floor!
I am not sure I understand how you are describing the box & floor attachment. If you can show a drawing or pic it would help.
When ya wanna stop by & see my TD ? Sorry I missed your last e-mail in time to set a visit up.
Ya'll be cool!

We are all travelers in this world , from the sweet grass to the packin house , birth till death , we travel between the eternities . ( Robert Duvall as Prentiss Ritter)