These brackets are designed to attach to the corner poles of wire racks such as that made by Trinity and sold at costco. The poles of this type of rack are 1" in diameter, with indents every 25mm. The back of this bracket is designed to sit on the pole and lock into place in the indents, then insert a zip tie in the slots and secure around the pole. Insert a piece of 1" dowel rod in each of the round slots on top of the bracket to form the spool rack.

The provided print profile has manual tree support enabled, only for the higher section of pole bracket. If you download the STL instead of the 3mf, you probably want to configure manual support, as without it the default normal support will cover nearly the entire part and be a pain to remove.