Hmm, You can get transparent green acrylic sheet off of Amazon (you can also use SendCutSend to get them to cut the acrylic into to the disc), and there are layers of plastic found inside flat-screen TVs and Computer monitors used to evenly diffuse light.
Alternatively, the lens could be 3D printed using translucent green PETG - the mottling caused by the extruder nozzle would probably work as a diffuser.
