While waiting for this feature to be added to the watch, here are other options:
https://www.thingiverse.com/thing:3528919
https://bikespot.es/accesorios-garmin/637-garmin-fenix-5-5plus-5x-plus-quick-release.html
Sorry, this is what you need https://buy.garmin.com/en-US/US/p/pn/010-11029-00
The bike mount for a fenix is different than the forerunner. The bike mount just creates a thicker mount for you to tighten the watch band onto. Only cost $15.00.
These are for the Fenix 6, which is why I suggested they might work with the 5+ - https://buy.garmin.com/en-NZ/NZ/p/pn/010-13013-00
Just need to compare the dimensions of the 5+ with the 6.