
I didnt want a permanent power socket, but liked the idea of having the ability to charge the phone when away camping, so I wanted something that didnt take up much space, and could be easily removed from the electrical system so it didnt have to be fully waterproofed etc.
This is what I made:

Part numbers on the tags
They also do a very small plug in converter to change the 12V to a USB socket
http://www.maplin.co.uk/Module.aspx?ModuleNo=274029
And a panel mounted socket if you wanted to make the power socket a permanent feature, saw it in the shop, but couldnt find it in a quick search of their web site. edit............part number A59FL
http://www.maplin.co.uk/Module.aspx?ModuleNo=99505
If you have a soldering iron it is a piece of piss to join the 2 together, only thing you have to watch out for is that the Optimate connector wiring convention is opposite to what Maplins planned, so the Black wire is the +ve, and the Red is the -ve. So black is connected to the center pin on the power socket, and red to the outer ring.