It's time to release your anger !
Open the old battery (I had screws). ACT WITH CARE : the battery, even weak, has some energy left and could arc with a watch, ring, bracelet ... and burn you.
Fit test :
Hack it up! :
I cut the edges keeping the electrical connector interface.
Glue the connector to the case and solder wires ;
We are done here ! Next comes the assembly.