Ed, it also might be a good idea to
put a brace at each end or corner of the tank, to keep it from shifting at all. That's easy to do: quarter round epoxied--or better yet, screwed down--to your shelf. If you brace the corners, miter the ends of the quarter round...it doesn't have to be 'finish carpentry" quality. I'm not a big fan of foaming in tanks. That boat builders do it doesn't mean it's a good idea. Boat builders don't care that eventually EVERYthing on a boat has to be replaced or needs work...they do what's easiest for them and has the least impact on their bottom line...and shooting foam that expands into a void is the cheapest way to support a tank. But it makes 'em much harder to remove--which is inevitable if a tank is aluminum--and can conceal a leak until the foam has become saturated with waste...turning a difficult enough project into a MAJOR one that starts with cutting all the foam out.