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!
Actually this is not quite true. To make a strong joint between the wall and the cross beam, I made the walls 3” taller than the doors, and screwed them firmly to the crossbeam. The decorative strips under the sign are thus 1 ½” narrower on the sides.
The walls themselves are made essentially the same way the back was done. The side edges were drilled to take furniture fasteners, and bolts driven through the arch protrusions to screw into them, thus joining the sidewalls to the arches.