My torsion box under my 24"x24" Primo is a 2x2. I used 3/4" mdf for the top, bottom, and all stringers. At the time I didn’t have a truck, so all parts had to be able to be cut from 4’x4’ pieces of MDF. This was back when the 525 version of the MPCNC was still the newest version.
Prior to that, my platform was a simple piece of OSB sitting on top of my table saw.
The entire torsion box built from 3/4" MDF is very heavy, and overkill. If I had to do it over, I might use 3/4" MDF for either the top or bottom, but I’d use 1/2" for the internal structure and sides and probably a 1/4" piece of plywood for the other top or bottom. I’m not sure of the benefits of having the thicker piece on top vs bottom.
I didn’t use any fasteners to hold the top on; simply relying on good joints and wood glue. My thought at the time was I didn’t want any metal near the work surface in the case the bit crashed into them… years later and I now have a spoilboard full of threaded inserts. Just goes to show that design ideas change.