A method I've seen repeatedly mentioned with respect to multihull modeling is to use blue board insulation foam. You glue layers of it together (roughly cut to the hull form), then sand it down to match the lines of the boat. Then vacuum bag a layer of glass over it. This can become a mold plug itself, or the hull. Depending on what needs to go inside the hull you can either scoop out areas of the foam, leave it all in, or use acetone to melt it all out.

The foam is supposed to be very easy to work and cheap to buy. If other structure is needed that can be accomodated (for instance, glueing thin ply between the foam blocks before fairing).

As you don't have as much time in a hull as you would from strip planking, you can more easily experiment with hull shapes.

Some foam may not be compatible with polyester resins, but it sounds like most are compatible with epoxies. If you're going to leave the foam in, you may not even need cloth on the covering, just a layer of resin to seal and fair.

Something else to try...