I built a frame for my Bridgeport out of 2x2x 1/4 angle. The angle is
welded as a frame just large enough to sit the mill in. I put 6
casters on it, they're 3 inch metal casters I got out of a local junk
store welded to the bottom of the frame. There are 1 inch bolts in
the corners for leveling screws. One person can move it around, but I
sure wouldn't want to have it on a slope. It's been in use for 8 or
10 years now.

Probably the only critical area is to be sure the caster won't
collapse under the load when moving. That could get pretty exciting.

Putting the casters outboard would be a good idea. If I were doing it
now I would probably do that which would keep the mill closer to the
floor by a few inches. I haven't had any problems, but mills tend to
be a little top heavy and extra insurance doesn't hurt.

Paul