I found an old Black&Decker electric screwdriver in my parents’ garage. The Ni-Cad cells inside had long since gone flat, but I felt it could still be useful so I decided to modernize it with some Li-Ion cells.
The cells came out of a defunct laptop battery pack(the laptop was dead for other reasons), and with some very slight modifications to the housing I was able to fit them in. The battery charging and protection was taken care of by a cheap battery management breakout board, but decided to bypass the output transistor for one with a higher current capability.
I also wired a larger RGB led to the status indicator LEDs on board, so that the original spot for the charge LED could still be used. I also wired the 5v input to the original charge terminals. The original charger is nowhere to be found, so I will have to make my own eventually. For now, I can just charge it by connecting 5v to the charge terminals using alligator clips.