A compost tumbler is the stomach of the compost digestion system. It aerates and mixes the compost so the the bacteria reach the new stuff quickly. This one is a barrel that pivots on a horizontal axle. Every time you throw in new compost, turn the thing over.
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

Wrap a piece of string around the middle of the barrel.
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.

Step 2: Drill the Pivot Holes

I used a 1" wood bit and a brace to drill the holes. It worked well. I drilled my holes an inch or so toward the top of the barrel. I did that so the barrel would sit right side up when it was empty.
The top will be a bit heavier than the bottom due to the door hinges etc. at the top.

Step 3: Cut the Top Off

I used a sawzall to do a plunge cut and cut the top of the barrel off. I left the lip of the barrel intact.
Save the part you cut off. That's going to be a lid.

Step 4: Lid Hinge Rod

The barrel top had a bit of a lip. I drilled holes for a steel hinge rod in the lip of the lid and the rim of the barrel. I though I'd need another to latch the lid shut, but I'd done a sloppy job cutting the top, and it stayed wedged closed with no additional hardware. It's also pretty easy to open. That was easy!

Step 5: Make the Pillars

If you have a convenient tree to hang your tumbler from, you can skip this step.
A couple of posts pounded into the ground will also work to support your tumbler's axle.

I had some pieces of welded square frame sitting around that were perfect, so I used them. I welded the U-shaped piece to the rectangular frame. I used the solar-powered golf cart welder with a spoolgun to do the welding. I used .030" fluxcore wire at 24 volts, the wirefeed was run on 18 volts.
I did 4 tack welds and was done. That was quick!

Step 6: Drill Axle Holes in the Pillars

If you're lazy a Vee notch at the top of the pillar is fine. Or a couple of nails pounded on either side of the axle atop a wooden post. Or of course suspending the whole thing by a rope from a tree branch.

I had the right size hole saw so I used that. I used plenty of oil so the cutter didn't clog or overheat.

Step 7: Install Axle Retaining Pins

Optional. I drilled holes in the axles, inserted nails, and bent the nails to keep them from falling out.
This will keep the axle from wiggling out of the supports.
This is the usual way I retain the wheels on a land yacht.

Step 8: Perforations!

Invite your frustrated friends to go nuts at the barrel with icepicks and drills.
I'll probably have to drill bigger holes than this to get proper airflow.

Step 9: Happy Composting!

That's it! You have a tumbler!
Keep your compost damp, turn the tumbler whenever you add stuff or need some entertainment. When the barrel is full dump it into the Pentagon Composter or put it on your garden.
Happy composting!