There have been a couple of l-o-n-g threads about pocket watches. In one of them I mentioned the leather watch cases that our local bus drivers used to wear on their belts and a little googling located some contemporary versions of those cases. Problem may be that most things aren't made with three inch belts in mind. Start thinking custom work...look around for a leather shop that does work for bikers and I'm sure that they can knock something up for you.

Best

AA