Hey!
I've got the same base as Dave. Only I turned mine 90 degrees, so the wheels don't stick out so much on the sides.
It's a bit bigger than the base of the jointer, as it was originally built for my bandsaw, but I swapped the base. I am in a small shop, so I need the jointer to have four swivel wheels so that it can pull straight out from the wall, and spin in place.
I'm gradually converting every mobile gizmo in my shop to have four swivel wheels. I have no use whatsoever (in a small shop!) for the two-fixed-plus-two-swivel arrangement. It is far too limiting and frustrating.
I've never bumped my feet on it either. Here is a close up of a wheel/corner, FWIW.
And yes, I noticed that the wheels are different.

I started out with four very nice 3" double-locking wheels from Woodcraft that I picked up on sale. Then when I built another cart I picked up a set of four cheap black castors that only single lock, and not as nicely. I then traded two for two with the base that had the nice woodcraft castors. That way I get two good double-locking wheels on each mobile base. I find that two locking wheels is plenty.
Cheers,
...art
ps: And I found this plan on the internet somewhere, it was from some guys personal web site, but I can't find it back to give you the link. Really, like Dave said, it's just a wooden tray suspended below a couple 2x4's. This gives you a cheap, versatile tool base without raising your tool up by much.