Perhaps instead of a roll-out set up you could do a fold out? I doubt you'd have enough length that a simple bi-fold would be long enough, maybe a z-fold (two folding hinged sections) would work.
If you built it as a frame that you could then fold out and lay cross-boards or other panel / fabric material on then you can use the void in the folding frame for storage.
Add some mounting posts at the front of the base frame so it can slip in to the seat-bracket mounting holes and a cross piece of flat stock so you can bolt it into the existing seat belt mounting hole with the seat belt bolt. Should make for easy, quick and secured temporary mounting. No more difficult that putting in and taking out the stock seat.
Just a thought.
Something like that.
Over lap a tarp over the roll bar / soft top and use a big tent-pole / guy wire set up out back for rain coverage. Or, weld some loops or mounting posts to the very most out-streatched part of the folding bed frame and set some posts in there when everything is folded out for tarp support. Probably would still need some rope or string to keep the tarp taught and sloped away from the bed.