I needed a shelf to hold my 1m level and long ruler in the garage, so I quickly modelled these C-shape brackets to do the job. For my purpose, I used 3 of them, but more or less would work just as well.

They are strong enough when printed in PLA. I used 25% infill and 4 perimeters (to strengthen the area around the screw holes).

While I was at it, I produced a variation that has usable ‘shelf’ area both sides of the support arm. Please see photos.