Here's the result using some parts from madvapes and a few otherwise-known-as-junk pieces that I can never bear to throw away.
I've even incorporated the original On/Off switch to serve as the master cut-off.
I used:
510 battery connector from madvapes
Tactile pushbutton switch, also from Madvapes
14500 Battery from, yeah, Madvapes
Brass strip conductor from an old flourescent light fixture (Told ya I don't throw anything away) to tie in the original switch.
About a quart, give or take, of hotmelt glue.
Sorry about no picture of the innards, but I was so impatient to get this fired up, I snapped the case halves back together too soon, and I was fearful that another attempt at disassembly would have broken some of the more delicate plastic catches inside. If it needs to be split open at some future date, there's always duct tape.