My friend and I have been considering carving a hull and structure out of foam for a moth, except use something better than styrofoam (it tends to soak up water over time).

There is a dedicated boat builders 5 axis mill nearby that make large scale moulds all the time. It should do the job.

Another little anecdote from a conference I attended on yacht design. The speaker had worked on a kiwi americas cup design many years ago when aluminium hulls were normal but the rules allowed fibreglass hulls. The rules were worded so that you could only build a fibreglass hull if it weighed the same as an aluminium hull. The whole point of building in fibreglass at the time was to reduce the weight of the hull so instead of doing the weight estimate for a standard aluminium construction, they designed an aluminium boat that was to be milled out of a solid billet of aluminium! This way they could have the most efficient aluminium structure with no weight for welds and the calculations could be based on the un-welded aluminium strength (which is higher than welded aluminium). I believe it was physically possible to do it at the time, although the cost would've been incredible! But they didn't need to actually do it, just prove that it was lighter so that they could build a fibreglass boat to that weight.