A bracket for holding planter boxes on the bottom of a deck railing to keep them from falling off.

The width from the inner square opening is 100mm which seems to be the standard gap between these black metal railings. The width of the square opening is 16mm. This is a snug fit so it wont come loose. 

I printed in PETG so it would last longer as it will be in the sun all summer, but PLA would probably be okay if you want to print them each season.