Rotating / Tumbling Composter
Here's the rotary composter I made a while ago mostly from recycled materials.
The compost is produced in a steel oil drum, which I've modified slightly by drilling a few ventilation holes in the lid and the base. Apparently it's important to use one which hasn't had anything noxious in it previously, i.e. one which didn't actually contain mineral oil at any point. Mine is/was a food grade one which originally contained organic pear juice according to the label on the side.
The frame is constructed from reclaimed timber: mostly tanalised pine 3x2s, with some lengths of other odd sizes used for bracing. The frame is deliberately made about 3" (75mm) higher at the front than at the back, causing the barrel to be tipped upwards slightly. This way as the barrel rotates, the material tends to move towards the other end.
The barrel rotates on wheels. The castors I've used are not ideal as they don't need to swivel, but they're what was available at the time. They're spaced so the strengthening rib in the side of the barrel acts against the wheels to stop it sliding backwards off the base.
Composter at work
Here's a snap of the contents of the barrel after about three months or so of operation. I add material (kitchen scraps, garden waste and the contents of the paper shredder) to the barrel every few days and give it a few turns each time after replacing the lid.
Not visible here is a sacrificial baffle in the end of the barrel made of a length of (untreated) wood wedged in against the sides which just helps to churn everything up when the composter is turned. This is now pretty black and I expect it will disappear completely before too long, but it's simple to make a new one, so I don't care about losing it.