Converting 5x AAA NiMH (6V) Device to Li-Ion Batteries: Best Options and BMS?

I have a device that uses a 9v power supply but also uses battery power as well, (5 aaa ni-mh: 6V). I would like to convert this over to use Li-Ion batteries. I know there needs to be some sort of battery managment in place as to not over charge the lithium batteries, I am just not sure what I will need. I also see where Lithium aaa batteries are more than 1.2v so exact battery count isnt a problem. Room is the only issue I have.  IT can fit up to 5 aaa batteries.. double a batteries are too wide.  If anyone has any idea or suggestion or even the correct way to do this easily, your input will be greatly appreciated.

Aronn, as you can still get AAA NiMh batteries new I think you are making extra work for yourself here.  Just replace the existing batteries!If you did want to update to Lithium batteries (and there may be good reasons for that, extra capacity being one) then as you say you will have to use battery management.2 LI batteries will give a voltage from 7.4 to 8.4 so make sure your device can work with that (put a couple of diodes in series to drop the voltage if necessary).  You can get pre-made boards for this, here's a cheap example:https://www.aliexpress.com/item/2S-20A-7-4V-8-4V-Li-ion-Lithium-Battery-18650-Charger-PCB-BMS-Protection-Board/32920006634.html
Given that you should have spare space inside the existing battery compartment, you may be able to fit the board in there.  This board is designed for 18650 batteries (much bigger than AAA) but you can use a resistor to current limit your charging source.

Thanks for the information.