This is my go at designing a simple bracket to hold a standard window box planter over a fence on my balcony. The smaller piece goes over my fence and I am able to print multiple of the bigger pieces to daisy chain / link them together. This results in being able to stack multiple  planters.

Print two of them and place one on either side of the planter to support it. It is suggested to print in a strong material like PETG or better. I need to redesign this to add more support on the lower portion of it to ensure it does not sag or break upon the weight of a filled planter box.