Simple 90 degree shelf bracket. 105mm long. Complete with angled screwdriver holes to you can get to screw the screw in! PZ6 screws will fit through the ‘screwdriver’ holes, bigger than that, place into the holes before putting the screwdriver through.

3+ walls, infill dependant on how much weight you're gong to put on it. I had 10% and it holds 2kg well enough. PLA, PETG, ABS. Raft not essential. Supports required for the holes.