Now is the time to focus on the electrics for the bike and making the mount for the milk jug.
I knocked up a solution that will house the battery, ignition switch, a couple of fused relays and the rectifier.
The battery I plan on using (4s3p A123) is tiny and powerful enough to run the starter motor, lights, etc. This will be located within the urn.
Here's a video of a similar version in action.
Here's some pics of the mocked up assembly showing the rectifier in position. It should still be able to get good air flow, whilst being protected (only just) by the chassis rails beneath the bike.