Here are the custom back rests/seats I made...
This backrest was made using two pieces of wood, some foam from an old dog bed my lab decided was a chew toy and some camo 1000 denier cordura material. I made the backrest only 8 inches wide so I could reach behind myself, to save on weight/materials/size. I cut the foam to fit one piece of wood, wrapped it in heavy duty plastic to waterproof it, then wrapped it in camo & stapeled it to the wood. The finishing piece of wood go screwed to the back and has small pieces of wood on the back so that it fits perfect as a seat or backrest and wont slide.
Here they are being used as seats
Here they are being used as backrests