I think we can run with what we've got, as there's no design more flexible than this. Accordingly although the spars are 20mm and the insulation board 25mm thick, we can still use it so long as the softest sides in the form of unprotected foam occupy the innermost parts of the boat viz. the inside edge of each pontoon and underside of the centre-deck.
It means the structures are not as rigid as they could be, but sufficiently so... which is all you want in a proof-of-concept. It will mean tho' that the pontoons are handed in this case, and eventually you'll want them to be interchangeable.
Here I've taken one of the longest spars at three metres and located it along the top edge of the sheet of plywood, with around 46" projecting forward to support the ski and 23" at the rear, to protect the 22" propeller disk in horizontal mode.
Then I've made a frame from the remaining spars before spraying the ply with sticky glue and setting it all in place. By way of 'belt and braces' there are screws driven in at either corner (as illustrated by the screwdriver). Finally as with all my signature dishes I've run a bead of silicone around the perimeter of the insulation board and filleted it with my forefinger.
Leave to set over night, along with the right-hand pontoon (this here being the left).
