Using a cheap skateboard and about $10 worth of hardware, I constructed this tabletop dolly. The best thing was that the only actual construction I had to do was drill one hole - no cutting, threading or welding. Depending on what type of plate you end up using (mine was a nice thick steel joining plate, already painted black), you may have to drill 3 holes.
The good thing about using skateboard trucks is that you can tighten the bolts really tightly, but because of the rubbers, you can still turn the trucks by hand.
There is some rough example footage at the end.