Just out of coureosity, has anyone thought of or used the styrofoam insulation panels rather than plywood or balsa? My guess is it would be as strong as balsa, not so much with plywood. But it would never absorb any more water and would be much easier to form to the cavity.
I've made plug molds with it using west system epoxy so the chemicals shouldn't dissolve the panel.
Foam is used by some builders. While it doesn't rot, any water that gets in travels much easier and the area of delamination is much greater. The advantage to end grain balsa is that any water travels pretty slowly, confining the damage.