Using the red Zebralight H501 as a bicycle bar end light

[Cemoi]

I'm looking for a brighter replacement for my Trek bicycle bar end lights, and thought that a Zebralight might do the job pretty well, if it fits inside my handlebar.
I'd be grateful if a (happy) ZL H501 owner could measure precisely the diameter of:

1. The tailcap
2. The body (narrower part)
3. The head (widest part)

Also, I wonder if it is possible (through a DIY modification) to make the beam more narrow, so that most light be directed towards car drivers ready to pass me, rather than spread over a 90 degree beam also lighting up the road, the sky and part of my body.

PS to mods: feel free to move this thread to the Bicycle section if you think it is more appropriate, but please not before the H501 experts have answered the two technical points above.

 

[longboat]

An alternative might be to have a small white-LED flashlight mounted so that it shines directly onto your rear red reflector - won't need much light (maybe moonlight mode) so is low power and lasts forever.

 

[MichaelW]

I wonder how well the new Quark prism retains its filters?
Because a quark + prism + red filter offers unique features.