Has a friend or family member ever given you a second hand bookshelf, including adjustable shelves, but they lost those little peg things that you put into the holes then rest the shelves on?  I know, annoying!  But you've got a 3D printer and you found this model.  You know what to do.

The FreeCAD file is included so you can adjust parameters.  The edges are chamfered especially the edge you press to get the peg inserted so that it feels comfortable to push hard.

Orient the model so that the long sloped face is face down on the build plate.

I used PLA with 0.2mm layers, 3 walls and 75% infill.