Today I assembled the internal grid and attached one of the skins.
I started by taking off the current workbench top, and leveling the top of the base. The top rests on two aprons that run end-to-end; one at the front and one at the back. I used hardwood shims (from a previous project) to adjust 3 of the legs and level the 2 apron pieces both left-to-right and to each other (front to back).
I then placed (2) 2×4s, which had been jointed and planed, on top of the aprons, the same as the torsion box will rest. I placed one of the 1/2” MDF skins on top of those to use as a surface to assembly the grid. I again checked that the skin was flat and level.
I later found that the middle of the MDF was sagging (only supported by the two 2×4 rails) and I then placed (4) 2×4 the other way to more fully support the MDF. It didn’t end up causing any problems (so far).
I started with the exterior 3/4” MDF pieces and joined those with glue and brad nails, keeping each joint square, to create the exterior rectangle of the torsion box. I then started gluing and nailing the (4) small 1/2” MDF pieces, and then the 1/2” MDF cross piece. I worked my way across the box in the same manner.
I used a speed square and some home-built squares to align each piece as I glued and nailed. However, the internal grid is not particularly square (each small square isn’t 90 at each corner). Luckily, that doesn’t seem to matter.
Finally the grid was complete. I tested each joint with my finger and used a block plane to knock down any height difference (just a couple of extremely light strokes). I then liberally spread glue on the whole thing, placed the 1/2” MDF skin on top and brad-nailed the top down along each row and outer edge.