I like a number of the features of the Lezyne KTV rear light, it is small, bright enough at night, USB recharges w/o the need for a cable, mounts real simple and easy, but unless it is mounted near vertical much of the light intensity is wasted.

I thought I'd pass on a mod which works well for me. One way the light can be mounted is with the included o-ring around the seatpost, but it aims at the angle of the post, downward. So I got out my dremel and sanding drum and ground an angled scallop at about 18 degrees at the diameter of the post.
Lezyne KTV rear mount-lezyne-1.jpg

Not perfectly vertical but much better and still simple. And it can still be used in clip-on mode for backpacks, etc.
Lezyne KTV rear mount-lezyne-2.jpg