If the end of the screw thread is narrow (or just the bare thread end), just make sure it points to the outer wall of the master cylinder, as opposed to pushing directly in the middle and thus on the weakest point etc. Leigh@RMS was kind enough to point this potential disaster out to me!

... or you could remove the flat end found on your standard 'mini G-clamp' (at any hardware store etc) and stick it on the end of the stopper thread for a more distributed load over the whole surface.