RE: glass shell??
All of the Airboats i build for my customer's are fiberglass . I first build a plug that look's like a finished boat, then apply mold release wax, then layup the fiberglass to make the mold . If you have waxed your plug well it will pop out ok.
went you lay up your boat the mold well need mold release wax on it to so it will pop out ok. So far as flat bottom boats most of the Airboat's i make are flat bottom.