The B&M Secula Permanent battery light for fenders, includes two AAA batteries.
Less than twice as expensive as the Dutch Fender Light, much better functioning, still a great deal. LED light provides a nice, bright, even beam.
A word from Peter White on mounting:
"Fender mount taillights require two holes drilled in the rear fender. The holes should be 5mm in diameter, one above the other, 20mm apart. Care should be taken to mount the light so that the brightest part of the beam is at driver's eye level. Where you mount the light along the curve of the fender determines where the beam is aimed."