Even the small Thumler barrels have flats inside their barrels. The flats offer some "grip" which reduces the amount the materials slide on the sides of the barrel; however, the important tumbling parameters are speed of rotation and the proper water/pin/brass mix.
If the speed and mix are right, you could tumble in steel paint cans in a home grown rig. A little bit of education in rock tumbling gave me a better understanding of the tumbling process.
A lack of planning on your part does not necessarily constitute an emergency on my part.