Thanks everyone for the responses! I should have been a little more specific. The new H. Tank is polypropelyne and is triangular in shape and rests in the V berth. Given the contours of a typical bow, the bottom of the tank is a smaller footprint than the top of the tank, as I took advantage of the hull flairing out as it rises, to make this holding tank larger (50-60gals!) So, its quite easy to secure the fore and aft sections of the tank as well as 2 aluminum rails on top of the tank to keep it secured down. But.... the port & starboard sides of the tank pose a more difficult situation. I could stuff firm/dense pieces of rubber down there... Foam came to mind and it occurred to me that I could line both the tank and the hull w/ plastic (say 4 mil) to asist in future removals if necessary, but perhaps cut out a series of holes in both the hull and tank sides of the plastic sheeting, so that there could be some adhesion to keep the foam in place, but making the cutting out quite easy down the road, with a hand held hack saw blade or what ever... yes? Thanks again JE I don't think I have to worry about crushing the H. Tank, as the foam would be quite free to rise up above the tank, as the V berth "deck" is out. I've been told that expansion foam will damage things, only if the foam has nowhere to go.