Exactly!
Re tubes inside the hulls, conventional car tubes will fit inside so I'll get a couple and once inflated, they might as well stay there for permanent flotation. However, as stated earlier, they will probably be too high to lift the stern as high as it needs to go for recovery. Please remember that unlike a kayak, this boat weight about 600Kg even without the water inside
The tractor tubes now purchased are too big to get through the hatch covers into the confined spaces at the stern BUT one of top of the other placed under the aft deck will provide massive lift right where it is needed. The only hassle will be getting them in place, then running the compressor line under the boat to inflate them - hence my need for a warmer diving suit than the wet-suit I already have
The only problems with straps is that even he 5m ones are costly and they are hard to get in 10+ metre lengths at any cost I can justify. Rope is cheap and the previous suggestion of placing a beam right across the stern will make it easy to fix in place as well as spreading the load.
Straps or ropes from the bow are doable but the fixings are not as strong