This is what I'm looking at currently:
It's about 10.5" wide by 6.5" long (not counting the reach of the spinner), which is probably a little on the small side. I'm prototyping the case with some scrap wood so we will see.
I'm trying to keep most of the weight on the non-spinner side of the wheels so that the see-saw effect keeps the spinner off the ground. I'm hoping to put a couple of caster-balls behind the spinner to help that.
Another nice aspect of this arrangement is the electronics are well-away from the spinner in case it goes (literally) off-the-rails. If weight and space allow I may put a wall between the spinner and the (very expensive!) weapon motor, too.