Looks quite well thought out, other than running a strap over the middle of where a 4x5" folding camera would be placed. To be doubly sure it won't fall out, even with the lip, which is a great touch. I cannot think of anything else to change.
While I'm not in the market for this kind of product, I think your ability to reinvent the carry case, tripod, and keep it off the ground completely, is possibly as ingenious as your SP445 tank.