Steve,
As Tami suggested it could very well be worth your while to talk to Dennis Palin.
He did design his own square.
He built it from 3mm okume ply sheathed in 4.7 oz carbon. I suppose you could call it a carbon hull , with a ply core instead of foam. Carbon beam and even a carbon striker.

I think the hulls came out at 52lbs each and very strong.

Dennis was good enought to allow me to post some construction pics and of course a couple of shots of the finished craft on my web site.
There is a pic of Dennis's creation in the attachment.

Good luck. I hope you build one.

Regards,
Phill