Not just mojo, but “Ultra” mojo!I'm sure the tolerances are pretty lax. I'm planning to open it up and poke around.
Edit: Okay I almost never got the door OFF again, LOL. So, nothing to see inside really (smd components and a mega328p), but I found if you loosen the two bottom screws slightly, the battery door fits better (on this one).
As for batteries, I don't really use them often, but when I do I make sure they are as mojo as well.