I used 1/2″ plywood on the bottom and with capet attached facing down. I have a tiled basement and this lets me move the bar around easily for cleaning etc. I built the 8′ kegger and even with two full kegs one person can move it around easily.
I built my 12 footer right on the concrete basement floor. I will be tiling the floor in front of and behind the bar. I just couldn’t see any point in putting anything under a 400 pound bar.