Get yourself 30' of 2x4, and cut six 24" pieces, and nine 21" pieces. Screw two of the 24's and 3 of the 21's together into a box, and top that with a 24" x 24" piece of 3/4" plywood. Repeat this process til you have 3 boxes. We use 3/4" stall mats in between the boxes, and one or two on top, depending on the depth required.