I was playing around with some red resin for my 3d printer and discovered that it can make a pretty acceptable tail light lens. Below are some lenses that I printed. The lens on the right is right off the printer. The slicing process makes a nice set of concentric circles on the lens, which is pretty realistic. The color is a little too pink though.
The center lens and the lens mounted in the bezel have a coat of Tamiya X27 transparent red on the back. It approximates the color of a real lens and really makes them pop.
If anyone has specific needs, I might try making something other than a circle.