I use a Cygolite Expilion, but any of the commonly available bike lights will work. Lezyne, Light and Motion, CatEye, PDW, etc... Mounting them upside down will mean having to reach underneath to work the switch for most of these. There may be models that have a rear mounted switch which would be easier to actuate. The switch often doubles as an indicator light, so that's a consideration too.

If you look at the business end of a light, circular symmetry with a centrally located LED emitter often always means a circular beam. LED lights that have sharp cutoff almost all employ an angled design where the LED is out of sight in the top of the light and the beam is produced by an angled reflector.