You can get a custom tank done or even buy the glass and do it yourself, but my suggestion would be to not do it at all.

ive had reef tanks for years and can tell you the wider the tank, the better! (Within reason, of course)

a 7" depth will be a pain to aquascape and take into consideration that a reef needs flow. A 57" long tank will need more than just a couple of power heads on each end. Lighting will also be an issue for such a tank.

12" depth would be the absolute minimum I would advise... personally I would not go less than 18". But if you end up going through with it, make a thread on reefcentral; I'd be interested in seeing how it all turns out!

They have one on Amazon