I built an internal coast to coast on my 120 (4'). Cost was very little as I drilled the top two corners of the back glass for 1.5" bulkheads. The overflow was built in a similar shape to a rain gutter with two pieces of glass so it only extends down about 5" and is about 3.5" wide at the bottom. The glass is tilted towards the front slightly so the water overflows down a gentle slope (sorry no pics). This arrangement keeps the bottom clear for live rock and more swimming room for the fish. Overflow can be thin glass as there is very little pressure.
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