Add a couple 2x4's in the corners and you'll be fine. The weight on the tanks will transfer through these 2x4's to the floor. Each corner should have two 2x4's butted up against each other in sort of an L shape so the can support each header (the side and front) if this is making any sense. After that, all you would need to do is dress it in 1/4" plywood just for covering. I just built one for my 120 and it could hold twice the weight. I used 2x6's for the headers but also didn't want to use a center support bar. I could of gotten away with 2x4's.