The base is several layers of 48" x 48" plywood screwed together. I used half of a ¾" sheet, a ½" sheet cut in two and half of a ½" MDO sheet for the top surface (to make it smooth), to make up the required 2 ¼" thickness. The bevelled sides were cut and mitred from 2x4s on the table saw, then glued and screwed in place. They each add an inch to the footprint, which is exactly matched by the signs at the top.

The result is very heavy, but doesn’t sound hollow or creak, which I think a hollow design would. Don’t drop it on your foot!