Here are some parts to build DIY batteries for Mavic Mini
http://aeropic.free.fr/B0B/

The most difficult part to build a DIY battery is the battery connector. If you don’t get the DJI genuine one, I found a way to DIY a female battery connector.
The genuine lyre of DJI battery connector offers 4 contacts to the male pin. The DIY battery connector will offer also 4 contacts per pin.
I simply extracted some female lyres from a 2mm pitch square pins connector and placed them two by two inside a 3D printed case.
It's a pain to solder in place and the best way is to use a male connector or at least a male pin to keep the pair of females in place while soldering.
Be patient, the result is excellent, we get the same four contacts to the male pin. Insertion into the male connector requires a lower but good enough amount of force than with the genuine one.
If the MM draws 10 A, this gives 5 A per pin as Vdd and GND are duplicated which turns into 2.5A per lyre and 1.25A per contact... It does a perfect job on B0B !

To make simple the battery soldering, use the 3D printed template. Print it and glue 2 pins inside the holes. Mark the polarities on the template to avoid any dramatic mistake.

Insert the female 8 pins connector, and put the cells in contact of the connector

Hot glue the 2 cells together and attach the connector to the cells with some strong double sided tape.
You can now solder your cells to the connector. Only 3 pins from both sides. The two middle pins shall be removed.

Don’t forget to solder the other side in serial and to add a one pin dupond connector to charge and balance the cells.

You can 3D print in spiral vase the B0B_center_batt thing. Just glue it under the cells, it will help centering correctly the battery inside the Mavic Mini hole. The conic slot is to be set at the opposite of the connector.

And finally You need also a way to secure your battery to the mavic mini : see MM_B0B_lock.stl