Here’s what I bought. The battery is nothing fancy and is something you’ll want to avoid getting wet since it doesn’t seem to have any sealing. There’s a smaller 3000mAh Talentcell pack that’s like $28 if you don’t need/want something this big. There’s probably other fancier options from other companies but I didn’t want to spend the time shopping for something else.
https://www.amazon.com/dp/B00ME3ZH7C
My light was already wired with a compatible DC connector, so I just needed this male-to-male extension to connect the battery. You may want a 5.5mm x 2.1mm long pigtail connector to wire to the light or something like this and a shorter pigtail.
https://www.amazon.com/dp/B09BQ81DSS
Chat about this stuff happened starting roughly after Fred’s post here: