Battery and charging solution

[BinaryBits]

So I have this LCR-T4 component tester that is driven by a 9V battery, I have printed a case for it and don't want to open it up every time the battery is depleted.
So what solutions are there to install a battery and a charging module instead of the currently used 9V battery?
Since usually the charging modules are only up to 4.2V, I need a 8-9V solution.

The physical size of the battery/batteries would need to be maximum the size of a single 18650 battery. And it would need to be a rechargeable solution.
Thanks in advance.

[Kim Christensen]

There's a thread about these testers on this blog. Here's a link to a post of someone doing exactly what you want. See their Thingiverse link:
https://www.eevblog.com/forum/testgear/show-us-your-$20-transistor-tester-enclosures-and-mods/msg1155862/#msg1155862

Another option is to buy one of those rechargeable lithium 9V batteries that have a usb plug on the side and then cut a small hole in the case to access it.

[BinaryBits]

Thanks for your answer. Yeah i also thought about the USB chargeable 9V batteries after I wrote the post. Only downside is they are ridiculously expensive, couldn't find them for less than like 25 USD....for a 9V battery.
I will check out the link you provided.

[tunk]

I've been using one of these together with an old, recycled
li-po cell. But, it is constantly discharged - either a high cell
discharge rate or through the module.
https://www.ebay.com/itm/114720734980?hash=item1ab5e37f04:g:oKoAAOSwf3pgSD1i