I built my platform by trial and error. Essentially I started with the idea that I wanted to carry a full size spare, and have space for several 8 gallon Rubbermaid boxes. Since these are both about 9" thick, nominal 10" pine boards worked well as the vertical supports.
I use good quality 1/2" plywood as the top. 3/4" is stiff enough to span the roughly 4' between supports, but is much heavier. But 1/2" combined with 5 boards is stiff enough. I use 2' wide panels rather than one 4' for ease of handling.
The next business was how to join the sides and top. I looked into angle brackets and such, but ended up using slotted steel and 1/4" hardware.
The initial length of 4' was determined by the ready availability of boards and steel in this length. It was accidental that length and position of the platform was just right to be used in conjunction with the lowered front seat backs.
In a much later step I realized that with the platform in place, I did not have easy access to the original compact spare tire well. So I made a replacement cover for the well, and filled the well with recovery gear.
paulj