I built one because the rich food waste that was going into the Pentagon composter was causing smell and fly problems. Hopefully the tumbler will solve that.
There are several instructables showing good ways to make a tumbler. There isn't a bad way to make one. I threw this one together in about an hour.
Step 1: Mark the Axle Pivot Holes
Mark the barrel and the cut the string so it just reaches around.
Fold the string in half and mark the midpoint.
Wrap it around the barrel again and mark the other side of the barrel where the string midpoint falls.
You now have two spots marked that are directly opposite each other.
Now measure down from the edge to each spot and make sure they are centered vertically as well as circumferentially.