A small collection of brackets; sizes as per filenames.
Once secured in place, you can tidy up the end result by printing the filler piece, either in the same colour or an alternative colour for a two-tone effect. The filler piece locks into place to prevent it falling out under gravity, and includes a small gap to assist with removal.
There's no special geometry here to worry about from a printing perspective. It should work for a wide variety of nozzle sizes and resolutions. My test prints were with a 0.4mm nozzle and a 0.3mm layer height. Clearance between filler and bracket is a total of 0.3mm in case you're curious.