This design is public domain and is available here on Youmagine.
I fitted a rear rack on my road bike and intend to use a trunk bag or garment bag there, both of which will block a taillight on my seat post, so I needed to mount my light elsewhere.
The rack has a good mount position, it's a standard reflector bracket.
The "reflector.stl" here will screw to a standard reflector bracket using the #8 screws in the BoM. The "pulseclip.stl" is a small clip that snugly holds an Axiom Pulse 60 taillight (I highly recommend this light BTW, great value for money and probably the best < $50 bicycle taillight on the market).
Note that there's no retention clip as such on this bracket, it does not "snap" into place, but it's quite snug and I feel it won't come out. If this becomes a problem I will redesign.
I designed them as separate parts to be glued together to simplify printing. The SCAD can be modified if you have different size requirements.
The pulse clip has integral support for the top rail when printing. Make sure to check your toolpath before printing and make sure the support layer prints with proper bridging. Slic3r didn't do it for me, I had to use Cura and cut down to 1 perimeter to get it to print right.
If using these two together, glue them together after confirming that they both fit your equipment. I used epoxy, Gorilla Glue would probably be fine too.
Note in the pictures I used a length of Spectra line to catch the light if it falls out. I really recommend a safety tether on your bike's equipment, I've lost > $100 in taillights over the years by having brackets break or just slip off.