Oceanic makes a thick-glassed 58 gal
tank with the same dimensions - just a bit higher. Maybe you can contact them about getting a top frame.
Another idea (not sure it would work, tho) is to wrap a piece of flat metal banding around the top and fasten it in place in the back. You know the metal strapping they use to secure cargo for transit? It's about 1/2 inch wide and you use a special tool to tighten, fasten, and cut it. http://www.uline.com/Class_09.asp
You might be able to have this done for a fee at a UPS, FedEX, or Mailboxes Ect location.