EDIT: I recreated this in better quality here: https://www.thingiverse.com/thing:5463325
I created this socket where you can mount your LEDs and glue them in the cover as it fits between the two mounting points.
If you use 3 LEDs per side (integrated brake in the taillight) then you can cut one socket and you have an easier fit.
Please be aware that even if the dimensions are accurate, depending on your printer you will probably have to use a 3 mm drill to fit the LEDs in.
History:
At first I drilled separate holes for the LEDs, but it was a pain to mount them from the back, also drilling the both sides simmetrically was a challenge. This part also provides better light isolation between the LEDs.
taillightv2.STL | 51.4KB |