I was able to build a battery pack using two three AA modules wired in parallel ( when you wire batteries in parallel they keep the same voltage but they sum their runtime at a given current ). I got a waterproof junction box to keep them safe outside and I mounted that on the back of the original light source case. I ran the wire through a hole between them and soldered them to the battery leads from the old flat rechargeable battery that was in the light originally which I had removed. The two boxes are screwed together with a couple of 3mm x 5mm (or so) screws with nuts and washers. I also sealed around the box with latex seal/adhesive so it is very secure. Here’s what it looks like inside:

So, as usual with engineering, simpler is better. I ran a test over night last night and the light source ran all night and into the early morning, plenty of time for what I want to do. When I get an interesting video I’ll post it.