That is a nice base. I was planning to get a mobile base for my x4 stand, but I believe I got an x3 stand, which doesn't have a rectangular base.
I ended up getting 4 of these leveling casters:
Great Lakes Leveling Caster
They threaded right in without modification. They are rated to handle the weight, so hopefully this will provide me with mobility when needed, as well as leveling capabilities.