I did not took any photos during this process, so instead, I've made some drawings to show how the holder was prepared during soldering of the wires on the board.

Prepare the surfaces to epoxy, by scratching them with a paper cutter and wiping it clean. This will ensure that the epoxy glue sticks well enough to the surface.

Put on the wires as described in the picture, leaving 1-2cm exposed on the bottom of the case. Prepare the wire ends for soldering, by pretinning it.

(8) Now insert the board back into the case, but do not fully insert it yet. Also bear in mind that solder points should face down, the reverse of what is shown in the picture.

Solder the battery holder wires, and finally, fully insert the board back into the case.

Put the cover back on. I needed a tiny piece of double tape to firmly hold the edges to the case, but your mileage may vary.

Finally, glue to battery holder to the case with epoxy.