The foam is called Plastazote LD24. It does not absorb water, is bendable and doesn´t break. It can be cut with a sharp knife, but best is a bandsaw, even a dull blade will cut it easily. Max recommended temperature range is 95 deg C or 203 F, so I assumed it´s good enough to park the airplane in the hot sun, at least with my reflective silver color on the plane. It does burn but slowly and is not really easy to get a fire started. Best it does not significantly expand when the pressure at altitude is decreasing.
I put in almost half a cubic meter which weighs 10 kg or 22 lbs. The weight of the foam is offset by the heavier salt water, that means I calculated the displacement as buoyancy - about 1060 lbs. I assumed that the airframe back to the trailing edge, the engine, prop and other equipment has another 300 lbs of buoyancy, so even when I lose one wingtip with 80 lbs of buoyancy while landing the airplane will float. Full tanks are good for another 90 lbs buoyancy, if they are emptied the things will get better.
I ordered 2 inch thick foam panels and cut out wing profiles like a wing rib. Cut them where the spar is, then cut them again lengthwise to be able to slide them in through the lightening holes in the wing ribs. The outermost layer I had to cut twice to get them in. It was partly just a trial fit and cut. Not all wing setions are filled as some are hard to reach.
The foam ist just laying in the wing sections, no glue or other fixation, just filled to get locked. Easy to pull out again.
I add some pictures so you will get the idea.
This is one layer behind the fuel tanks with a cutout for the wiring and aileron pushrod.