The downside of my battery solution is they are quite easy to steal. Real e-bike batteries seem to have locking mechanisms.

My approach to address this is "security by obscurity". I designed and 3D printed a battery enclosure that encapsulates the batteries to make it less obvious what is there. It requires a screw driver and some time to get around or some bashing to free the batteries. My hope is that in the rare short instances the bike is left unattended and locked up this will be enough.

The enclosure is ventilated to keep the batteries cooler. Next is to design and print a mesh style end cap for the enclosure so you can't see the batteries from the top side. It was left open for wires to exit on the backside and to shorten the print time requiring less support.