Very neat, and lots of info on a small display. Tidy installation too.
The battery issue could be solved with a small battery from a burglar alarm. They are readily available, sealed and can be mounted at any angle. Although they only have small terminals, you won't be drawing much power to run this display. It's not like you need to fire-up a starter motor from the battery.
Something like a 3.2Ah 12v jobbie like this:
http://www.yuasabatteries.co.uk/np32-12-yuasa-9-p.asp at about a tenner, or check out the data sheet for all their NP range (including physical dimensions) here:
http://www.yuasabatteries.co.uk/data-sh ... ds-4-w.asp
If you want 4.8mm terminals and a crimping tool to fit them, then I have plenty in stock.