What about this idea:
cut out the corner flow box and use the holes that are already drilled into the bottom of the tank!
Since there are two holes in there, I could seal the "return" line hole with a bulkhead fitting very easily, and just have the pump gravity fed off the drain hole (with some sort of strainer of course) and just have the pump underneath the tank itself! Voilia, no need to drill anything! And the cornerflow box/cover could be re-sealed in later if I sell the tank down the road!
Any foreseeable issues?