To Combine two boxes, you need two M5x60mm hexagon socket head cap screws, 4 washers 1mm thick, 10 OD, 5.5 ID, two M5 nuts and the two printed spacers.

The top is closed via 2 round magnets, 10 mm OD and 3 mm thick. I glued them in with hotglue.

If you dont want your batteries jumping around inside, put a bit of paper or something else with a bit of flexibility into the bottom. I used some sort of gluable gasket/seal for doors.