Well, we did try to cut the plywood so that the larger peices would have a good look to it, and the peices would kind of match, within the limitations of the sheets we had.
The nice thing about the 4" I-beam is that a 2X4 fits into the I perfectly with a tight fit. We still bolted the 2X4 to the beam, which gave is the wood surface to use for attaching the cabinet. A 2X4 was also bolted to the underside of the I, and a 2X4 was attached to the floor with tap con screws.
The plywood is was not really screwed on to the beams... but is held by some adhesive we picked up at lowes. the styrofoam is glued to the plywood with the same adhesive. I dont remember the name... but I tried to pick one that I felt would not chemically react with the styrofoam over time.
The one problem we ran into is that the I beams had slight roll to them after welding, and the floor was not even. When we put the tank on the stand.. the tank was perflectly level but there were some gaps between the plywood and the steel beams. So while the tank was level sitting on the stand, somehow the stand was not. When we tried to shim the stand.. the tank was no longer level and the gap was still there between the plywood and the steel. I guess the overdesigned steel stand was unable to deform with the shimming. So we went ahead and shimmed the gaps between the plywood and the steel to take care of the problem.
sanjay.