B&M makes several that mount that way.
4DToplight Permanent $26.50
(has an on-off switch)
4DToplight Senso $36.
(on-off switch and auto on sensor)
The two lights look the same and only differ in their switches.
Peter White carries both.
Link--about 3/4 down the page.
Make sure you get the battery powered versions and not the dynamo versions.