Here's the battery, shaved about a mm down each side to fit into the seat mast (I filed along the join, down to the groove, didn't go through the casing), with a couple of tiny holes drilled in the square bit on top, where I've got a bit of wire rigged up to orient the thing for removal. Kevlar string attached to that, wound around some rolled-up bubble wrap.
This lot gets stuffed down the seat tube as far as possible with a large poking implement of some sort, and the end of the kevlar is secured in the top of the seat mast with a small blob of blutack.
Ta-daa! Pretty much finished. Just gotta take it for a spin and dial in my position (I've swapped a 17/110 for this 8/100 stem), fit a little button onto those bare wires, and get some fresh bar tape on there.
I'm pretty stoked with the result
Any questions?