One thing about your design... I'd probably suggest a full lip around the freeze tanks where they go into the main tank with 'ears' or something to lock them in place. The lip will give you a place to put a line of sealant to make a gasket on each freeze tank or on the main tank.

I'll see if I can whip something up to show you ... I hope you haven't started cutting the pieces yet...

Lastly... how thick is your material? 1/8 Inch? 1/4 Inch?