I got my hands on the LiPo Battery Gauge mod that Alex from insideGadgets has designed. Check it out in action here: https://youtu.be/iv5W8DdY6K0
Want to build your own? Here is the repository with all the required files: https://github.com/insidegadgets/LiPo-Battery-Gauge
Do note that BOM (Bill of Materials) is inside the EAGLE schematic files for the PCBs but I'll sum everything up for you: 10x 10k SMT 0805 resistors (or 0603, both fit fine), 10x 1206 SMT LEDs in whatever color you want, 3x 0603 0.1uF capacitors, 1x 10K 0603 resistor (R6), 1x 320K 0603 resistor (R7), 1x 1M 0603 resistor (R2), and 1x ATTINY48 VQFN-28 package.
QFP ATTINY48 AVRs will not fit and you'll have to use a custom board. If that's all you have, you can use my board: https://www.dropbox.com/s/w7s3ttzasrlnpj2/iG-Lipo-Battery-Guage-MB.zip?dl=1 but do note that my board is larger to account for the larger AVR (the firmware is the exact same for both AVRs).
Using oshpark for the LED board portion? You may need to make it bigger. I added a mm to the top of the board in eagle and will upload that as well (it still fits but now it meets oshpark's minimum board size requirements) (will upload soon).
And finally, if you only have a 300K resistor instead of a 320K resistor, you may need to use modified firmware. I have tweaked the firmware myself and recompiled it. I'm unsure of how accurate it may be as the battery gets lower and it will require testing. I'll report back on that one but for now, here is the firmare I made: https://www.dropbox.com/s/73zbhj40wwxuu3r/lipo12a.hex?dl=1
Regarding flashing the firmware, the following is what I did. If you are having troubles with this step and what I did does not work for you, I cannot assist you.
If you are using a USBasp and are on Windows, you'll want zadig to install the libusb-win32 driver specifically or avrdude won't recognize your hardware. Once setup, you'll need to download avrdude and the main.hex file from the insideGadgets repo and run the following command: .\avrdude.exe -c usbasp -p attiny48 -u -U flash:w:main.hex and you'll probably have to add some lines to your avrdude.conf file just to get it flash an attiny48. This worked for me: https://lists.nongnu.org/archive/html/avrdude-dev/2009-08/msg00005.html
Just add the text after "avrdude config entry" to the end of your avrdude.conf file.