Building the Platform...

To save money, my original idea was to use a piece of 3/4" plywood covered with three layers of carpet pad and then covered with another piece of 3/4" plywood for my platform. I thought this would be strong enough because of the cushion the pad created, and I wouldn't have to buy any extra lumber to beef it up. I got the carpet pad for free from a local carpet store and even if you did have to buy it the price is around .17 cents/sq ft for "good enough" pad for this type of project. Really nice carpet pad can cost .90 cents/sq ft but for this project i didn't think that necessary. After constructing the box this way I decided it wasn't sturdy enough though and instead used plywood, 2x12's and then more plywood stacked into a platform. Fortunately for my wallet, I ended up having enough spare 2x12 left to go between the plywood and beef it up sufficiently (still sitting at $164 for the project!!).

After cutting the 3/4" plywood layers, I screwed 20" piece 2x4's to the bottom of the plywood as tabs that would prevent the platform from bouncing around when the weights are dropped. Then I laid down two 40" 2x12 planks side by side on the first bottom layer of plywood. From there the top piece of plywood is screwed down on the 2x12's.