After spending a fair while trying to get a JoyCon strap replacement I liked the closest I found was Po1nt's.
I took his design & modified it a little:
I printed this using 0.08mm layers on a Bambu X1C & it works a treat.
A quick point of note, when putting this onto the joy con have it so the buttons are both facing down, this will prevent the pins interfering with the rail.
The author remixed this model.