I successfully cut out the old molded in tank on our Hunter 31. I left enough of a lip from the lid to place a plywood lid (re-enforced with 1x2) to fill the v-berth platform. I then created a frame out of 2x4 lumber and glassed it to the hull below that opening (after coating with epoxy). On that frame I screwed a plywood platform that is big enough for a 13 gallon tank. This is a little smaller than the original, but for my use I really need is a weekend of capacity. The platform has a frame that is the shape of the tank so the base of the tank is held in place. I then used 1" ratcheting straps to tie the tank down to the frame. I had to add a 1 foot extention (left over from when I replaced the hose) to the vent hose and will probably replace that hose in the coming year. So we now have a holding tank again. I flushed the hull to remove any spillage from the hoses that got past me when I pulled them. Things seem be smelling much better. On inspection I can see that the fill hose connection was not completely glassed to the tank or was cracked some time in the past. But even without that issue the chalked joint on the lid was smelling and would definitely do so again in years to come even if I re-chalked it. The rest of the summer will tell if this work has paid off. Thanks to Peggy and others for comments, suggestions, and just plain listening. Next, the water heater.