Ended up purchasing an heavily discounted Fenix 5 instead of a Forerunner as planned, not realizing that there is no quick release for the Fenix. Looks like the "official" solution is to use a universal mount, i.e. something looking like a wrist unto which you strap the watch.

Any alternative?

Your arm?

You probably want something like that:

I just wear mine.

Maybe I should add that the watch will be used instead of a bike computer, and to navigate on tour.

Simply wearing is absolutely fine if the purpose is to acquire data, but I've found it impractical for navigation. Which is why my Forerunner was mounted on an out-front mount, via a quick release adapter. Easier to keep an eye on the route, and possible to change the display while keeping one hand on the bar.

The mount suggested above is what I was referring to as the "official" solution.

Raceware offers something that is closer to what I had in mind.

It's yours set up to vibrate on your wrist when a turn is coming? That's pretty obvious and hard to miss. When you're navigating, the a page that shows an arrow with the direction you need to be going, plus the distance to your next turn. I use mine to navigate bike routes in unfamiliar places, it sounds less than ideal but in practice it works fine.

I don't have advice on mounting options, you might use foam or something. The screen isn't very large, and it's easier to see in motion on the bike when you can pull it closer to your eyes.