Hi ninsei,
necrocia reached out and offered that I should do a guide or howto on Atari Gamer but I havent had the time to respond much less create the guide.
To help you with your mod, the advice Id give is:
1. Wiring and placement - when you disassemble the ONN power bank, separate the battery from the board. The battery needs to fit under the screen in the old battery compartment area and the board should be glued/mounted/affixed at the top of the Lynx mothernboard. I ran wires throughout that connected the ONN board, Lynx, battery, and switch. Sorry no diagram here.
2. Getting power from ONN board - remove/desolder the big USB Type A port from the ONN board. Thats where You get the 5V output to shoot to the C41 point (via power switch). Look at a USB Type A pinout or use multimeter to see correct points.
3. Dremel use - shave away on the ONN board around the Micro USB port so that it protrudes enough to be easily accessible through brightness wheel hole. Also, you may or may not need to slightly enlarge hole with power button. Depends on if you use a switch cap and if so, how big it is.
4. Switch cap - I had a hard time getting it to fit onto the switch (tight). Eventually, I had to trim the switchs nib down and I added a tiny dot of glue just in case.
Good luck!! Its been working well for me. Let us know you experience and if you can improve on this mod!!!