Thanks @chrisys, I’ve added PiJuice to my wishlist, but as said by @zestyping on Running a Fin on USB or other battery power? the board is hard to find right now. 
As I’ve said in the same post, S.USV could be another option. LiFePo4wered/Pi seems also to be interesting.
Another uncommon feature on RPi could be a power button and maybe a use button too. I know I can use a Grove module for that, but maybe adding some buttons on the DIN-rail case would be nicer.