Another alternative might be to get a Forerunner 305 instead of an Edge. I don't know for sure about that model, but with my 301 (which doesn't have a cadence option), the wrist strap doesn't need to be removed from the unit for it to snap into the handlebar mount. Then I could just fasten the wrist strap loosely around the handlebar as a backup in case it fell out of the mount (which it never did).

The drawback is the display - monochrome and smaller versus the Edge's large color display. You also wouldn't have the option to import topo maps as I think you can with the Edge 705 (or street maps, but that wouldn't matter for the trail).