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Just guessing but,
Port Plastics and others sell sheets, BUT!, the problem is a 2x3? or 3x4 sheet min, will set you back 40-60 bucks and that will not complete a 40-50gallon tank,(WITH 1 sheet), so there is then 2 sheets needed and Weld On 16/3or4 and that is 6-7-8 bucks a tube and then getting it cut to size and so on, picking it up and all.
I would figure you would be better off looking for a used 40-50 gallon tank or maybe even Wal Mart. The used tanks off these type sites, are the best deals and no hassle.
If it is just a Sump, how's about the Home Depot and there storage containers? You can plumb them togeather and make quite a large Vat/Sump system. I know many are using the 22 gallon size vats, for sump needs.
Good luck,
M&J T :nod:]