I prefer that lights like Xeno E03 and Trustfire R5-A3 have a clip that permits wearing on the bill of a baseball cap as a headlamp as well as pocket or belt carry. R5-A3 has a clip but it points the wrong way for a cap.
I ordered clips from Fenix gear, one PD10 which fits the R5-A3 and one LD20 which fits the E03. Both clips have a nominal ID of .9 inch, however the PD10 clip is too tight for the EO3 and fits the R5-A3 perfectly. LD20 fits the EO3 perfectly.
The clips are $1.50 each and shipping for both in one envelope is $1.
Mine are always put on tip pointing down from the head, I do not waste money on separate headlamps that way....they fit an ordinary cap.
I've done that with other lights. To be honest I'm not sure why I didn't do so with the E03.
On the E03 the clip shows the writing at the top of the flashlight betwen the cutout in back and fits 90 degrees off from the finger grooves. Perfect
A bit unrelated but ...one of those two pictured is a replacement for another E03 that was faulty. On the one I returned the finger grooves were 180 degrees from the writing rather than 90. I had completely forgotten about that until you mentioned it.