Bit of an update. I've now made the frame, wired up the motor and attached a bolt to the bucket. Here's a couple pics of what I've got so far:
The motor and transformer. The wires will be shortened once everything is mounted up.
The frame. Two wheels at front support the front of the bucket and help it rotate, the rear holds the ass end up to avoid placing excess stress on the motor.
Underside of frame with rubber feet.
Bolt through the bottom of bucket. It's hard to see but there's a rubber grommet on there and on the inside, I'll also silicon around them to provide a seal.
Length of stiff polymer rod used to connect the bolt and motor. It's been threaded and will mate the two parts together like a drive shafts.
Some shaped plastic I found. I'll attach a couple of these to the inside of the bucket to act as fins to agitate the brass and media.
I've still got to build a frame to mount the motor to so it's the correct height behind the bucket and then it's pretty much ready to go.