When I installed sliding pull-out drawers in my IVAR shelves, I needed something to keep the back of the shelf from pulling up out of the metal pins holding it into the uprights.  This shelf stop has tapered pins to fit snugly into the upright holes and lock the shelf into place.

Print with the diagonal face down; this will not require supports, but will print the pins at an angle for added strength.  I suggest using the “Strength” profile and printing out of PETG.

Here is the Onshape source document: link