The dimensions are:
11.5" Deep at ends, 16" at center
Now it's a tall tank. I am considering whether or not to just scrap it and buy a rectangular tank (larger but not so deep). But if I can find a good lighting unit for it then I would rather drill this one and make this one work.
Oh - it has a brace in the middle of the top - which I expect a single MH centered would melt through.