Hey Murph.
I wouldn't cut teeth into the overflow. I personally like the smooth edge on the overflow better. You get better surface skimming, and will get less turbulence in the box. If you're worried about critters getting in I'm sure Keith could rig you up a slick cover, or you could make one very simple out of eggcrate.
You only need a 3 sided box. Glass holes makes a 4 sided box, and drill holes through the box using the bulkheads to anchor the box in place. I am NOT a fan of doing it this way... I just think it's unnecessary, and opens up a whole new can of worms... you need to make sure the box is level and you cut all your holes correct to keep the box level.
I cut PVC to the height I needed to support the box in place, and use clamps to hold it there during the install.
You're going to want the top edge of the overflow box to be located 1/4" to 1/2" below the trim line on the tank. This will make sure the water line is hidden from view, and also that your water line is not too high in the tank.
Once you have the PVC cut to the correct height, I clamped everything into place dry, and traced around the outside of the box, so I had an estimate of where the edges would come into contact with the back wall.
Then I unclamped everything and ran a bead of silicone inside the traced line. Clamped it tight to the back wall then quickly ran a bead on the inside of the box, and outside of the box and smoothed them out. Quick check to make sure everything stayed level, then let it set for 24-48hrs.
*If you want all your visible silicone lines to be neat and straight... I use blue painter's tape to mark everything off, and leave like a 1/4" to 1/2" gap between the tape and the outside edges of the overflow. Then when you smear the silicone, any excess will get onto the tape, and when you remove the tape, you get a nice clean line. You just have to remove the tape IMMEDIATELY after you finish smearing all the joints."
