I think 120gl will be to small. My buddy and I our building one right now. I also I you math might be off just a little on the grate size. the bottom ones are 27by22 and there are 2. the top ones are 27by20. (the tops could have been bigger) but that still only gives us 1188 for the bottoms and 1080 for the tops. together 2268 sq in.