I made mine also using #200 drawer slides. The first time I built it about 5 years ago I used 1x4 pt that I had laying around and a piece of old countertop from a remodel job. That lasted about 4 years then fell apart. I took the whole thing down to the weld shop and they fabbed a heavy duty metal pan and bolted it to the slides. Then they bolted some 3" aluminum angle to it and I screwed that to the floor. Works fantastic.
here is the new one