i do not know of any current manufacturer that has a non tempered 55. I have one that i wanted to drill and I did not know the maker of the tank so I called, marineland, perfecto, glasscages, aqueon, and top line and all of those are tempered on all sides. Perfecto was the only company that said it offered a non tempered option for the back panel only. And that had to be special ordered. Because the glass was thicker. From what I gathered talking to them as well as some more knowledgeable people on here is that if the 55g is made from 1/4'' glass it is tempered.----------------------------------------------------------------------------------------------So OP just buy a peice of 1/4'' glass in the size you need. 48x12 in 1/4 will not be more than 50 bucks max. And a 48x21x12 tank will not hold 2 rays for long.