Before you spend a lot of time figuring how you will build it, have you figured out how you will keep it cool? I have a 16 gal with a 5.5 (about the same volume as you) sump and a 175 MH SE in a pendent 8 inches above the water and I still had a little heat issue. I had to buy a chiller
Can you use a pendent, or does it HAVE to be a hood?