For a drilled tank, the box idea don't work too well.
My suggestion is to get a piece of plexi the same height as your tank, cut slots at the top, and bend it to a semi-circle or a 90 degree elbow (depending on if the holes are centered or at the corner) and silicone it to the tank at the sides & bottom ... like this one:
125 Gallon, need some plumbing suggestions
80 gal FW with 30 gal DIY wet/dry/sump.
9 fancy golds, 1 hillstream loaches, 1 rubber-lip pleco (C. thomasi), 3 SAEs, small school of white cloud minnows, planted.