I've got a set with smaller steel wheels and the metal strap from front to back like mentioned above- got them at a swap meet for $100 iirc. They work fine on SMOOTH concrete- suck something fierce over cracks, grooves, dirty floors, small pieces of gravel or screws, etc. They also have a tendency to twist as I'm moving the car around and can squirt out if you're not careful. I've thought about larger rubber wheels, but never seem to get around to it.
Another warning- BE SURE TO PUSH ON A HARD PART OF THE CAR.. bumpers, corners, etc, and DO NOT PUSH ON FLAT SURFACES!!! Somebody pushed my GT350 back into the corner by pressing against the fender just above and back of the front wheel. Now I've got a nice smooth dent there. I'd probably kick his *** if it weren't my own!!!