Perhaps if you go down the keel and bulkheads route it might be better to make the bulkheads undersize to allow to plank the surface for a mould plug, rather than fill in between the bulkheads with foam or balsa.

I think planking sanded smooth would give you a better uninterupted surface than filling between the bulkheads and then sanding smooth.

However, either way could see you with a lot of work to get the surface you're after for moulding a display model and it might be better to just carve from solid.

Anyway I wish you luck and hope you'll post the results here.