Summary

It should be stated clearly up front, that since this is technically a safety device type item, I cannot guarantee this is up to any codes, regulations, or even technically safe to use.
Because If it is made incorrectly based on material or thickness, people will not be able to see you stopping clearly!!!
I release this as a reference design that anyone CAN use if they want, but need to make clear it's used AT YOUR OWN RISK.

It's working great for me so far, which is why I wanted to share the designs but I don't want to be liable for any injury because of someone else failing to make this correctly for personal use and it resulted in injury.

I needed a new lens for my Honda rebel clone Tail-light. No clue what the official tail-light shroud is, but if someone else can print and use it then it doubled the usefulness of it! :D

Printed in semi-transparent magenta PETG (came out a little pink however when in use). DO NOT USE PLA! Incandescent bulbs get hot, and will easily melting PLA)

I suggest printing with thick WALLS, but THIN top/bottom (creates a more transparent lens area for the light)

I used 1.2mm walls (3 passes) with only about 0.3-0.6 top/bottom thickness and 10% infill. The ultra thin lense uses only two passes at .175 layer height to maximize transparency.

Note: I haven't tested PETG's ability to last near a incandescent bulb. But know PLA would melt with use.

UPDATES:
7/19/17

• Added a Thinner lens area for both the version with and without the lip. allows more light through than original design. and I may add a even thinner version after this. It is a Tail-LIGHT after all and visibility is absolutely critical otherwise it's a dangerous piece of plastic when people can't see your brake-light!!!

• 7/25/17 After more testing I determined the original design was way too thick to be safe. I have removed it and instead added an even THINNER lensed version. This should only have two passes over the lense area to allow as much light through as possible. I am not sure how durable PETG will be against OEM bulb heat but I plan to switch to LED soon so this is the final version I'll release unless there's another reason to edit this.

USE AT YOUR OWN RISK! THIS IS SAFETY EQUIPMENT AND FAILURE TO MAKE SURE PEOPLE SEE YOU STOPPING IS A SERIOUS HAZARD!

I release this as a reference design specifically, and Cannot and will not guarantee it's up to any codes, regulations, or safety standards. Use at your own risk!