I have never seen a muller with horizontal wheels (3), but here is an example: http://www.aapfoundryequipment.com/1145-b-p-100b-muller-awo3356.html
That's pretty neat... Making a backyard version would require special attention to wall construction obviously. I could might be able to get way with doing it, but I'd be concerned about the slide gate I have in the wall. It runs true enough, but I didn't give any consideration to pressure that would be applied to it in this kind of app. I would love to see someone take a build like this on though.