We set up one of the spare wheels to bolt flat on the A frame behind the stone protector, and bolted a tool box above that with clearance to remove the spare. Then riveted aluminium angles to the toolbox top to sit the chemical dunny in it, and hinged a clip down frame to stop it bouncing out. The ITPBITB (Impossible To Put Back In The Bag) ensuite tent slips in behind that for fast roadside breaks.
All this worked fine until the sun killed the dunny bag, but that will be an easy fix.
I did hope thieving kids would think it was the fridge sitting up there, and try dunking for coldies, but no luck with that yet.
Cheers