I was not quick enough to buy a genuine Jolla battery so I tried with batteries of an old Nokia phone, ref. BL-5C (850 mAh). Not very convenient but it's working. I will keep this dirty hack in my bag, just in case.

The battery needs to be adjusted a little bit (need to cut a small part) and it's a little thicker than the Jolla one but nevertheless, the cover can be easily closed.

In red, the location of the plastic wedges I made to secure this small battery.