OK, now I understood, my blame I do not catch it before.

Then what you propose is safe enough after you set a voltage near 4V.

I like a lot more what you done in the mentioned thread because I do not want to compromise to much portability.

What kind of material you used to add that white (black painted) layer to double the battery space?

I reproduced it with modelling foam, later I hardened with cyanoacrylate but the results is not as desired (pictures later).

I would like to 3D print it, but first how to model it? Somebody has a 3D file of that?