I've made one. I think I was the first to make one with this method. I cannot take credit for the idea and most of the (not yet public) documentation though. That belongs to HoZy. For the most part, you just have to reroute power (battery input, power switch, 5v to system, audio power), audio output, and controls. It's tedious to do for sure but actually pretty simple. The key, I think, is to desolder the cart reader from the GBC before trimming and remove the cross brace under it (flush cutters or a dremel) so that you can route wire under it. You have to run quite a few wires. Additionally, you'll want to remove the high voltage LCD voltage supply capacitor or disable the high voltage output on the regulator (by removing the pin). Leaving it connected is potentially dangerous (in the exploding capacitors way).
I'd recommend studying up on the schematics for both systems or it won't make too much sense doing this.
Pocket:
https://github.com/Gekkio/gb-schematics
Color:
https://nintenfo.github.io/repository/systems/GBC/documentation/schematics/