I created an old battery cap for the Nikon EN-EL15 battery about 8 months ago, and this is an update. It is a dual sided cap to both protect the battery connections as well as allow you to easily determine weather the battery is charged as you can put the cap on so the battery is on the FLAT or CHARGED side.
Update Jul 2018 - I have redesigned the file in Fusion 360, and made it fit a bit tighter. The main reason for the upgrade was to put FLAT and CHARGED on both sides of the case, so I took the chance to upgrade it a little in the process.
0.2mm is fine, but the letters might be better if you go down to 0.1mm resolution. Manual supports in the middle of the underside will help stop sagging of the first layer of filament, but keep them 1mm away from the edges or you may find the battery does not fit easily