I managed to lose the little rubber grommet thing that covers the battery screws on my Ninebot One S2, so I made a 3D printable copy. Posting incase anyone else runs into the same problem and has a 3D printer handy.
You can grab the files + some more details on Thingiverse: https://www.thingiverse.com/thing:2783786
I printed it out of a rubbery TPU and it seems to do the job!