Next is plywood for the sides, back, and the bottom shelf. Used birch, Home Depot's cheap C-3 grade. The 2x4 structure was already rigid, but added panels seem to tighten up the stand another notch.
The sides are nailed on with 2 inch finishing nails, the back is fastened with those 3.5 inch deck screws.
Notches were cut in the plywood for the bottom shelf to fit around the outside front and back leg pieces, then I used 2 inch finishing nails in just a few spots to hold it down. I seem to have missed taking a photo, but a second panel of 1/2" plywood covers the 2x4 on the inside. Just happen to have enough from a previous project that I didn't have to buy another sheet. That's about 50 pounds of plywood, this thing is getting heavy.