3x18650 battery holder, with slots for battery contacts and for a BMS at the top. Because the unit will be under shelter but outdoors, I've added the groove to the shadow-line step feature, and the slope on the top, and the holes on the bottom surface, in order to manage water ingress.

The shitty AA battery contacts I bought aren't the best though, the one without the spring is tough to get any clearance with, it means the retaining walls have to be pretty thin.