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.