something to consider Ron.
since you have some broken masts why not think seriously about that old mast mini

We dont know much about the torsion qualities of windsurfer masts, but i suspect it would be a bit poor , so why not go for a frame like the one Ive drawn,
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You probably wouldnt need to use carbon , just woven cloth and epoxy resin. you could incorporate a glassed in floor thus making , as hiko said

, a monocoque chassis. It would also incorporate your seat.

by keeping to the mini size , even with layed over wheels you would have enough ground clearance, thus fulfilling gizmos piont worth remembering

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this would be a really tough, but really lightweight design.
there are some of us that would contemplate doing just this if we had the time and a few more broken masts

I would stick to the LLM dimensions, as you seem to building a yacht that will be a handful to sail, and probably wont give the perfomance you say you would like