I did a bunch of research on this a couple months ago, and as far as building your own, it just isn't price efficient. It's one of those things where unless you are buying the glass and other materials that are needed in bulk, you will end up paying as much or more than a store bought tank.
The only type of tank that I have seen built, that MAY fit to what you like is the kind of tank where all the sidse except the front viewing side is wood that is covered in a coating of epoxy sealant. This is usually used in an in-wall application where the tank is set into the wall and the front pain of glass is the only viewing plane anyways.
Another reason to steer clear of larger DIY tank is that if you want to cut glass yourself to the correct demensions, the glass you are using will not be tempered (unless the glass is already the correct size) because as soon as you cut, nick or scratch tempered glass, it loses it's temper and is only as strong as regular window glass of the same thickness.
I wish that I would have had better news, but I tried all the options that I could think of and I will be going to a lot of garage sales this summer to try and get a good deal on a large used tank!
Good luck in your endeavors, if I find the DIY page about the in-wall aquarium I will post the link.
/www.duboisi.com/diy/BNdiytank/bndiytank.htm
