Stay away from the bar xtender from topeak. I had one and it snapped with just a 2AA light and my air zound on it (though the road was a bit bumpy). I now use a pair of
Dashboard Genies to mount my bag (and my light and my airzound) and still have room on my bars for my computer and interupter brake levers.
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Bikes: 1996 Eddy Merckx Titanium EX, 1989/90 Colnago Super(issimo?) Piu(?), 1990 Concorde Aquila(hit by car while riding), others in build queue "when I get the time"