Can't quite see how this one mounts, but a little fabricating if all else fails should do it:
http://www.ascycles.com/detail.aspx?ID=1310Looks like some promising leads here too:
http://www.thealtimeterstore.com/Also, if you're just after functionality instead of aesthetics, most better GPS can give you altitude readings too (My Zumo 550 does, I loved watching it on my ride across Colorado)