I have two versions of the same part. Both should be easy and fast to hang on existing shelves. I designed it so it can be hooked ANYWHERE. You should not have problems, I designed so it will not interfeer with any spool on the shelve.

Most important, it will not try to twist the wood so no extra force will be added to your brakets.

Just hook and go.

Simple version should be easy to print, no supports, one pice, and should be strong enough to hold anything you put on it.

V2 has a lock mechanism just in case the Simple version has a balance problem and falls without a spool. This requires some assembly, M4 screw and locknut, and a rubber band.
Optional I've designed pin to replace the M4 screw.