I use Garmin watches since ever (see here) and I finally found the solution on how to mount the watch on the handlebar of the bike! This is also a very cheap and easy solution!
You can find all the necessary stuff inside the Triathlon bundle box. In particular you need the quick release kit. The problem is that wearing the watch 24h/7 I'm not comfortable with such a big tool. I don't like its appereance too. The easy solution is to buy a watch strap with a quick release machanism. This make the removing of the straps easyer and faster and you can link the watch to the bike in seconds.
You can buy a original Garmin's watch strap for 50 euros or a non original one for less then 10 euros.... it's your choice!
Since I'm curious, I weighed all the stuff composing this solution (watch without straps and quick release kit handlebar mount) and I was surprised to see the result! Only 58 grams! Take in mind that the watch with its straps weight 51 grams!!