Just a thought but can you just build a wood box and line it with a styrofoam cooler which is easier to find in larger sizes and cheaper? Or perhaps line the box with a heavier gauge vinyl such as on old waterbed bladder or something similar. Not sure if you want it to be removable or not. I know the big azz coolers you are thinking of. Used them on the coast for big springs but they're damn pricey and it's unlikely anyone will part with one. Hope you find one though.
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Aquaholic
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