I don't have one, but the general feedback on glass cages that I have heard is that they make solid tanks, but sometimes can be a bit sloppy with the silicone, and some other details, but their prices are good. Here's the vendor experience threads on them that I found;
http://www.bostonreefers.org/forums/showthread.php?t=39201&highlight=glass+cages
http://www.bostonreefers.org/forums/showthread.php?t=20719&highlight=glass+cages
This place sells complete kits for drilling and installing your own overflows. I hear good things about them as well.
http://www.glass-holes.com/main.sc
Also, last I heard you can order custom sizes and overflows through AGA/aquagon, but you have to do it through a dealer (basically any sponsor LFS). IIRC they can be pretty expensive for custom tanks, but just adding an overflow in a stock size tank may not be too bad. Check with a LFS on this (I'd suggest CRA and or skiptons to talk to).
Last, custom DIY set ups are possible. I've done them for others in the past, but I'm kind of booked up at the moment. This thread might inspire some ideas if your handy;
http://www.bostonreefers.org/forums/showthread.php?t=78520