2020 Update: Added a version with a through hole on the lower mounting point for easier access to the screw while hanging. Either way, I use a regular screw driver to put the screws in the wall after punching the holes with a power drill.
I took a bit (a lot) of inspiration from Joel Telling's spool mounting bracket, but I had a 2x2 lying around instead of a 1x2. Also, I used this project as an opportunity to mess around with the simulation tools in Fusion 360. Results pictured with 98N (10kg) of load assuming solid PETG. Not bad! I actually printed it in PLA, but the two brackets share the load so it should be well within the safety zone.
Be sure to use a heavy duty mounting screw on the upper screw hole (like a toggle bolt) if you're mounting to drywall. Overall, everything looks good and shows no sign of failure under load.