Motor Dolly suggestions
Heres another one if you want more design ideas to make one.
http://www.arengineering.com/engine/motordolly/smallblock/motordolly.html
Its stable and when not in use takes up little space. A little pricey to buy, but looks pretty simple to make. I have one in use right now and the casters roll really smooth. I only have one because I used to be a distributor for ARE.