I remixed this nice corner shelf to include holes for wall mounted screws. I also resized it to be a bit bigger to fit a speaker I wanted to put on it. Lastly, I carved a hole in the back of the shelf (in the corner) to allow passage of wires for the speaker. The dimensions of the shelf, back corner to first screw hole, then to the next screw hole are on the attached picture. Dimensions of the screw holes are the same on both sides.

The stl is oriented upside down so you don't need supports. The flat side goes up once printed. Mount the screws on your wall (level), place the shelf slightly above the screws and slide the shelf down onto the screws. 20% infill is plenty.

Anycubic

Large Size Chiron

20