I sent you the PM because I think this will be of little interest to nearly all who read this section.

If you can find a case to fit it, the simplest way is to buy a two cell battery holder from overseas. Then find or make a case to go around it. If you put the batteries and case in a plastic bag, the camera bag which I use and which attaches directly to a bike frame should be the simplest way to go.
2 cell battery holder for 18650 batteries – 89 cents postpaid from Hong Kong https://www.ebay.com/itm/ABS-

A pair of the 3,400 mAh genuine NCR/Panasonic batteries costs around $12 on eBay. They work far better than the Chinese batteries with the fictitious mAh capacities that don't really exist for this size battery.

Here is a picture of the small camera case that I use to carry the battery case. See the two types of holders – pre-wired with spiral clips to hold the batteries. Do not buy that type as they do not hold the batteries secure enough to keep your light lit. The flat clips work far better.