the ortlieb mount is fine. i leave the mount for mine on my flat mountain bike handlebar... even when i got offroad, and of course i use it during tours. adds a wee bit of weight, but then again, what's the heaviest part of a bicycle?... i'd bet it's the engine...

don't sweat it, just get one that's secure and leave it there. you'd never know the sheer convenience of clipping on the handlebar bag for an overnighter or something close to that...