Very clever. But how much??? A mate of mine out in oz has a double cab pick up, one of those mitsubishi L200's I think but its got a semi rigid tent that folds away completely in the flat bed part but then folds out and over the front cabs and down behind the truck, loads of space. think I'd go for that option and spend the spare twenty grand on jammy dodgers or something.
OK for a VW, but no good for a Commer. The G forces created by a Commer accelerating would cause the back to keep popping out. The only consolation would be that you wouldn't get the reverse effect with Commer brakes!
Besides, the Commer has enough usable interior length already.