The kind of tumbler I've seen before seemed like a fairly simple arrangement - some kind of barrel turned by horizontal rollers, similar to a tumble dryer, so that's what I set out to build.
Step 1: Parts List
Parts list - these parts all came from my junk pile. Nothing was purchased for this project, so technically it cost $0 to build. As suggested by several people (iceng and ottwafm), the tumbler would be more durable if some kind of wheel bearings were used for the rollers. Bearings for skate board wheels or ride-on mower decks should be easy to find locally.
1) electric motor from Grand Am electric window lifter.
2) old PC power supply. This one happens to be a 90W supply.
3) rollers - could be broom handle or any round pole. These are actually mini rolling pins from my wife's craft supplies.
4) various bits of wood to make a frame and base board.
5) 2 inches of garden hose.
6) power switch.
7) screws, coach bolts w/ nuts
8) anti-slip shelf liner
3) Drill + appropriate sized bits
5) Sharp knife
6) Soldering iron (or use crimp-on spade connectors)