I'm installing a 25 gal roto-molded Ronco holding tank in the v-berth area, just forward of the aft most bulkhead. The tank has been made to somewhat follow the shape of the hull. So as to evenly support the 200 or so possible pounds that the tank may weigh when full, I'm thinking of placing the tank where I want it and then spraying the 1"+ Great Stuff expanding closed cell foam all around the underside of the tank. Additionally, so as to not 'glue' the tank and foam to the hull I'm thinking of first laying down some mylar (aka 6 mil visquene) or large 30 gal style trash bags so as to create a mold release. I'd also tape this same plastic to the tank. After the foam has cured, I hope to lift out the tank and remove the foam plug mold and the hull lined plastic. Then I'll cut away where I need to for my outlet hose in the mold plug and just set the mold plug back into the hull. Then I'd install the tank with an aluminum flat bar bracket holding the tank to the bulkhead.
I've sprayed some of the Great Stuff onto different plastics including a piece of wood coated with PVA on a part and vaseline on another part. The Great Stuff released pretty easily from the mylar and garbage bag there best, next best was the vaseline (will make a mess).
What does everyone think of this idea? If / when, some water or effluent gets in and or around the foam will it cause Mould and will I be able to clean it up? It seems to me, that having a uniform light weight mold is far better than some marine ply or heavy fiberglass matting. I expect the thickest (depth) of required foam would be about 4-5".
Thanks in advance for any suggestions or constructive thoughts.
Attached is a rendering of the hull section and tank used for fitting purposes.