Mostly they have cast iron bases so they sit hard on the bench, but you could just pick a heavy sandbag you like and build a U-frame around it. As long as the bag was filled with something dense, you wouldn't lack for weight. Actual sand might do it.
I expect you can figure out adjustable feet for yourself, but if you expect to make one-wheel windage and elevation adjustments much less a joystick control for it, then you're going to be into metal parts or at least some hefty threaded inserts. Basically its an X-Y table rotated 90° to the bench so Y becomes Z, plus feet, and with a bag sitting on its top edge.
Bottom line: no, I haven't run into existing plans for such a thing.