In my latest signal design iteration I split the signal head to facilitate inserting the SMD LEDs I got from my friend David at WeHonest on eBay. The red-yellow-green LED were pre-wired at his facility in China.
the lens is 3D printed as well as a mini-Fresnel lens to fit flat into the signal head and to get the light focal point the same as a traditional LED. The lens has a rectangular opening to insert the SMD LEDs in the back of the lens.
the back looks like this
here is what the wired lens look like before insertion inti the signal heads.