After the band loop of my Garmin vivoactive 4 broke, I design and printed this replacement.

The part is rigid, however is very comfortable due to its thin profile and rounded edges.
There is no internal retainer, however the fit creates the right amount of friction to keep the loop in place, while allowing the band to be removed.

The loop is also sized slightly smaller than the buckle, so it cannot fall off (you need to detach the band from the watch case to install the loop).

I have worn this part daily for about 5 months now and it is still going strong.