I used Dave Tates instructions... if you look, you can find them on his site but...
2x4 "frame" 24'x24" on edge. 1 cross piece in the middle dividing the box in half.
24"x24"x1/2" regular plywood top. I made 5 boxes and screwed them together. We have done heavy partial dl's off them as well as box squatting with NO DAMAGE. Use wood screws. No extra bracing needed.
You can buy 6'x4'x3/4" rubber "stable mats" at a horse/cattle supply or grain store for $40.00/ea. cut into 24x24" squares and use them for micro-adjusting of hieght. Or just use the two leftover plywood "tops" for the same purpose (they DO slide, though).