Extra rechargable battery behind the speaker

i was wondering if it is a viable option to install one rechargeable battery on the bottom of the gameshell where there is empty space and solder it to the main battery. Has anyone done that or can point to the right direction on how to do it? I’m not sure what kind of circuitry is needed , if any.

if i’m not mistaken the size of a rechargeable battery is 6,6cm x 1,8cm (3400mah) , not sure if it fits though since i don’t have my gameshell with me to measure it.

I was initially thinking about an external power bank as a solution but i wouldn’t want to affect the portable form factor of the gameshell and besides i think an extra 3400mah battery is ok for a handheld of this size.