bud, I recently built one for my in laws for Christmas. It was 4'X8'.
I took a 5/8" diameter all thread, that was cut to 5" long. drilled my hole in the lower 2x4 runner, and placed one nut and washer on inside, one nut and washer on the out side of the 2x4, then the caster would slide on to the all thread, and place another nut on after that. I also located the holes for near center of gravity balance. I used pneumatic casters, since they would be removable.
So you just used the pneumatic tire rather than the entire caster assembly? Like a hand truck wheel?