I have rescued some baby joggers from the trash over the years. The wheels have an axle sticking out one side of the hub, which you stick into a tube on the stroller, and then there's a spring to hold them on. They're easy to pop on and off. I suggest you find a couple those wheels and find a tube of the right size. 1/2" copper plumbing pipe is the right size, and I'm sure that's not the only option. Anyway, mount the tube permanently to your 'cello case, and pop the wheels on when needed. You will need to figure out how to keep the wheels from popping off when they shouldn't, but that shouldn't be hard. A set screw, for example, would do nicely.