Making your own will work great if you use quality casters per Howard's suggestion. However, I don't agree that you should skip the locks on the casters. High-quality casters on a smooth floor will roll very easily. In my state, garage floors must have a certain pitch towards the door and with high-quality casters the car will roll out of the garage. Even if your floor is dead flat, you don't what the car moving when you're installing a transaxle, sanding, whatever.
I second Gojaks. They are great for moving the car around. There is no need to use your floor jack and the car's CG is low -- helpful if you're rolling the car out of the garage and you need to negotiate some transitions etc. They're low profile, so I just roll them under a bottom shelf when not in use.
I use 10" tall wheel cribs from Race Ramps when I want to work on the car with the suspension loaded. When I'm working on the car, I don't have any concern that it will roll. In addition, they're very light and easy to toss on top of a cabinet when not in use.