i designed a small shelf bracket where you can add your favourite type on shelf material

i made mine from pla with 10% infill and 4 walls and easely withstand all my daily violence

it has a 9 mm hole so any screw smalle can go through with a fitting nut, i used a washer to make it hold up better when you tighten the screw (mine is very tight) 

i added a small piece of plastic to go up against the cauch back so the screw does not punch through