I use a Klick Fix mount, and their wire support frame allows other bags to work too,
the same mount for a Detours or their KF version of a cloth front basket, oval wire support , It has a drawstring to keep stuff in .
wider oval and bigger than my Ortlieb [ they're 10" wide] bag.

But both will leave a mounting clip on the bars,
but its screwed on so wont get stolen separately.

this rack on Quill style stems line Nitto, or Cinelli 1A, is removable
but then you need a different bag..
http://www.velo-orange.com/rackaleur.html

traditional handlebar bag will un buckle from that rack,
http://www.wallbike.com/berthoud/ber...dlebarbags.htm