Have you tried these?
https://www.speedwaymotors.com/Techn...hoCp1MQAvD_BwE
I'm not crazy about how these look and prefer the glass lenses but if you going to make lenses anyways this seems like a simple solution.
The real problem is that they are small.......and low. If the car doesn't already have a 3rd brake light, I highly recommend it.
Lenses are typically machined from acrylic and then candy painted on the backside, then you also machine a diffuser from the same material.