Don't use leather!!!
For every Rollei I have had, and other cameras with a leather strap, I have tried to break the strap at the join with the metal fastening thingie - On some occasions this has taken a bit of force, but I have managed it - The metal thingies are than taken to a boot repair place with a length of nylon strap pre-cut to the required length and they rivet it on
Breaking the leather strap on purpose is vastly better than gravity doing it for you while clambering over rocks - OK, not a purist approach, but my cameras are for use, not to look pretty - Even when they are
Don't trust Rollei leather straps