Have you ever considered using a fake back panel in your tank that acts as the overflow?
For example, the physical dimension of my tank is 30" deep, but the display area is only 24" deep. When looking at my tank from the front it looks like there is no overflow when in fact, what you see as the back wall of the tank is in fact the overflow. So my external overflow box is
(the height of the tank) x (length of the tank) x 6"W.
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