How about something like this except make it as a watch strap? You could just replace your strap and not make your watch thicker (my pet peeve when it comes to watches). You could probably even have a larger battery and have it last for several days. Most watch straps are standard and either 18mm or 20mm. You'd also be sure it wouldn't fall off - I know they say the chronos won't fall off but come on, many people aren't going to trust micro suction.