Hi Mike, I have one of those and I believe that you can find them for less in some aliexpress/ebay stores. Very similar model to the one reviewed here There are a few variants that you could get in the chinese lottery though, my unit has the switch in the lamp head and not in the box but the box definitively contains the diver and charging/pcb chips (and also a LED in most units).
It is not still clear to me what your priorities are for your application, but it seems that you only need the box. If that is the case you could consider a cheap plastic hermetic box (I think these are large enough to squeeze the batteries and even a holder or a PCB inside, I have some and I can measure the inside for you). The problem for mounting these into a strap is that you would have to figure out a pouch or something that could harbor the box and hold it to a head mount.