I've had really good luck with the
Synergy Overflow, you just drill 2 holes on the back wall (or one of the side walls). Other companies make them too, but this one is super thin inside the tank. Eshopps makes another really slim one,
see it here. The slim profile is super nice because it doesn't cast shadows down the wall of the tank, and frees up more real estate for your rock work and corals!
I installed
one of these in my old one, and the shadow went halfway down the wall and I couldn't get GSP to grow there while it was covering the rest of the wall! The shadowing is real!