Modeling a battery holder for gameboy

[Nigan]

Hello everyone,,
So I've modded a Gameboy pocket with a USB port and rechargeable battery. The battey itself is quite big, so it won't fit in the battery compartment. My idea is to 3d print a battery holder that I can weld to the back of the gameboy. I plan to showbox.bio/ tutuapp.uno/ vidmate.vet/ possibly allow for the battery to slide in and out. My main concern is clearance. Cell phone batteries aren't all that thick, but when modelling, do I need to add a little space, like, say, a millimeter to ensure that the battery will still slide in and out,or (should I take the permanent solution) at least slip in the first time? I don't have the original phone that the battery came from to use as a guide