Quick update; I acquired a dense foam yoga block and cut it longitudinally before attaching it to the skateboard, with a gap for the drysuit inflator to nestle into. This makes it really comfortable to lie on and ride, with all the heavy gear on. (I might stick some duct tape around the blocks, for extra security.)
One forum member (Ladymud) kindly offered to supply a buoyancy aid; yes please, this would still be extremely useful as we hope to set up a couple more boards for this use.
For
Bob Cromer; no-one else has be able to sort out any spare boards, so could I take you up on your kind offer above? The optimum length of the "deck" (as I believe it's called) is 30 inches.