Following the photos you posted the iron frame rests directly over the 2x6 frame.
So,,, 2x6 frame, 3/4" plywood, Hardiback cement/fibered structural backing board and ceramic tile,,,,
park a truck on it and it won't crush that thing LOL.
The Fraternity of Dirt
If at first you don't succeed,,, keep kicking it
2-75g planted, 5-55g planted, 5-20g planted, 110g w/30g sump, 8-10g,
2012 update adding table top pleco pans & a 90g (Nutz)