As Dcarr said, 3/4" plywood ripped lengthwise. The 3/4" pieces I glued & screwed together to form a 1-1/2" solid wood countertop. That's what I chose to do, for a 8' x 2' workbench top, and then got a piece of 3/16" steel sheet cut to size to top it. I use mineral oil to wipe it clean & to keep it from rusting. Held to the wood & frame w/flathead 1/4" machine screws, countersunk. The steel top is edged with 1-1/2" X 1/8" steel strap screwed to the plywood.
I used a Craftsman 8' x 2' steel frame I bought new, $88 out the door, it normally sells for >$200. The rollers under the workbench I got at Sears, they are Craftsman door or drawer units, selling for >$400 each, retail, I got for $84 each on a closeout.