I recently received a custom built tank (for a bimac octopus) from www.glasscages.com, and I had a mixed experience with them. I highly recommend that you TRY to get a tank from them, but I suggest that you go about it carefully:
1) Their website is great, and gives you a clear idea of what they can build and what it might cost.
2) The tank is great. Thicker glass than most other tanks of the same size, built to my specs.
3) The price was unbeatably low
4) The delivery cost was only $50 for a HEAVY 75 gallon tank from Tennessee to Los Angeles California. Amazing!
5) They built it in a day or two.
6) They also build great oak stands for a great price.
The people you have to work with, are a little crazy, and not in a good way. Remember "The Soup Nazi" from the TV show "Sinefeld"? They all seemed to have a chip on their shoulder, and to be just itching to find an excuse to say "No tank for you!" At one point they told me they had decided not to build my tank. There had been no friction or rudeness, they are just very very touchy. I think they have some strange power issues, and just like the way if feels to push customers around. Since I couldn't afford to have anybody else build my tank, I aproached them again a day later, this time in a humble, gateful, subservient way, with a less precise, dumbed down, design for my tank. They were happy to do it this time, and I got the tank. There were a few things about my tank (hole placement, size, etc.) that I was literally afraid to specify precisely, for fear that I might trigger another "No tank for you!" response. As a result there's a big ugly hole in the wrong place on the wall of my tank, and a couple of little things that aren't the way I would have liked. The tank was still worth the price, but it's a shame it had to fall short of what just becasue of personality issues.
I think that they build great tanks for a crazy low price, but they are also a little crazy themselves. I think that if you treat them as if they are smarter and more important than you, and as if you are grateful for their wisdom and willingness to help you, then you'll have no problems and get a great tank at a great price. I looked around hard, and couldn't find a way to get a custom tank without paying almost double.