Is it possible to do a foam support?
Wrap plastic sheeting around the tank; immerse it into a throw-away outer
box/form. Spray construction foam ( gingerly ) into the "form" until the
entire tank is supported on all sides, as deep as you want the support. I'm
not sure if the foam is closed cell and it may or may not absorb water over
time, though. Just a thought.
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RichG .. exclude the x x when replying