We have had a number of enquiries from mountain bike riders and cyclists who want to have ViewRanger on their handlebars so that it can be used in a way similar to an in-car satnav.
First of all, there is a setting in ViewRanger to disable the phone's screensave function. This will allow your handset to show ViewRanger's screen display continuously. (Battery life is obviously reduced, but we've had Nokia phones run for 11 hours whilst navigating a route with screen always on and a bluetooth gps receiver connected).
Secondly, you may want to have a look at the RAM Mounts range of mounting solutions. In particular, the splash/mud proof AQ3 case has a clear and flexible cover so that you can see/operate your phone handset without removing it from the case.
There are several tube mount options - but the RAP-274 Rail Mount is possibly the smallest.
Check out http://www.ram-mount-uk.com for more details and locations of dealers.