Drill out the back, and do it right. Corner overflows, and bottom drilled tanks are absolete. Better overflow designs, and siphon drains systems, have made the 'reef ready tank' something not to be desired.
Without an overflow, you get no surface skimming, no surface renewal; gas exchange and 'protein' skimming suffer. These are very important aspects of the marine aquarium system. Don't set up any drain system without an overflow. (The open end of a pipe, or upturned elbow is not an overflow by any definition.)
The C2C overflow, doesn't go to the bottom of the tank, so there is no need to be drilling the bottom. If drilling 2 holes out the back for a 2-pipe siphon system (herbie's modifictaion is a waste for anything other than a corner overflow,) drill three holes and use Bean's system. Why settle for so so...