The tank, I got from Glass Cages. Not to pull this thread off line here, but I should comment on this.
All I had Glass Cages do was build me the tank. I didn't want them to drill holes or do anything other than make a glass box. The way I see it talking with many friends who have ordered from them is that simpler your order is, the more likely it would be carried out exactly as planned. As it becomes more intricate, you are less likely to get what you wanted and in some cases (Brett) get nothing at all.
As for their work... you get what you pay for. It's very sloppy with a lot of stray silicone patches I had to scrape off. The glass is scratched in 2 places. If I was going to make this a reef, I would not have accepted the tank. But knowing that the turtles are going to put some scratches in the glass anyway, I was able to be much more forgiving. IMHO, I would not use a glass cages tank for a reef. They also failed to provide me with a turtle shelf which I ordered and was on my bill. They did credit my card back for the cost of the shelf.
I'm not well experienced working with glass. I've drilled holes and cut glass, but I never tried to make and external overflow in the middle of a back pane of glass. I lacked the proper tools and did a sloppy job. I'm not sure if I would have been better off if I had GC do it, but overall, what I did works and cannot be seen.
What I did was drill 2 holes and connected the dots. If I were to do it again, I would have gotten a drill press to keep the hole perfectly in line (the one on the left dropped a little while I made my initial groove) and I would try to better with the diamond bits. I burned 2 of them and then had to use a rotozip diamond bit attached to a cordless drill which doesn't have anywhere close to the RPM's a roto has. I went off line with my sawing motion.