I had exactly the same problem t'other day with my clutch cylinder and some Harris clipons. By far the easiest solution was to chamfer the edges of both the clipon and the master cylinder. Still isn't perfect but it's miles better. The m/cyl base has a surprisingly think wall (although you obviously wouldn't want to find out how thick!)
Otherwise you'd be looking at a s/h m/cyl off a 916 or similar. Anything with a remote resevoir should do the trick.
Cheers,
Ali