Instead of a closed loop are you open to the idea of running powerheads or wave makers in your tank? A pair of wave boxes on one end would work pretty well in your situation. Penductors/eductors would also work well for you too but I think either way you still need some sort of output at the opposite end of the tank or else you might get a dead spot...
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32"x32"x20" Cube-ish tank
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