I have a wave box on my 120 and love the look. The coral sway back and forth. Its not a rolling wave, but I think its close. An exturnal wave box would be great, they take up a lot of space inside a tank.
If you make the box about the same size, and glue it on the outside I don't see any problem. Just make the box the same hight as the top of the tank.
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120 G sps reef, looking to build bigger. 
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