According to the sticky, and if the measurements were done correctly, two M1 cells are 1mm (52mm), yes 1mm too wide for the compartment. (51mm)
Thiessen, you said 1/2 inch, I really am confused here now....
Then if I look at the lid, from what I can see another 8mm of headspace in itself. So, wouldn't 4s pack fit in each side? Am I making a mistake somewhere?