Just a question is there a reason you would like a 2" gap all the way around? I would suggest 2x2x1/8 square tubing as the structure but with that we would have to put a Sheet of metal over the top to catch the supports if you could live with 1" gap it would be a better stand in my opinion but it's just a question so I know how to price out metal
Just my oppinion but this seems a little overkill. I have built several metal stands now and for a 200g 1.5"x1.5" 11 guage (1/8th") is more than sufficient. Also I have never used a metal sheet over the cross braces. I just make sure that there is a cross brace about every 16" or so and just use 5/8-3/4 plywood over the top. The plywood will act as cushion of sorts for any minor imperfections in the structure.
Also keep in mind the dimensions. If you end up going with something too wide it wont fit through the door. This does not seem the case here as he said 24" x 84" but something to keep in mind as I made this very simple mistake once. This is a nice solution for two pieces I designed for another person. The Idea came from a stand that AGE built for one of their large tank customers.
Again not saying my way is the best just my experience. On a stand this size you will save allot weight and money by not going overboard on materials.