The casting part is easy. I have cast lenses and many other parts. The issue is getting the correct polyester so that it will have any decent type of lifespan and proper clarity. The UV is murder on liquid casting polyester. The yalso change shape too readily in heat.
Most of the automotive lenses are actually polystyrene or a variant of it. The ones that yellow too quickly are made of SAN (styrene acrylonitrile). But SAN has better impact strength. Its a tough call.
Styrene and SAN are not usually cast. They are generally injection molded.
But on simple parts you can heat Polystyrene pellets and vacuum mold them, but the mold required is combination of high temperature plaster and silicone. You will need to heat the mold to at least 2/3 the temp of the acrylic. The acrylic pellets must be melted at a rather exact temperature so it does not burn or oxidize...then be poured into the vents of the mold. The mold is kept hot so that the styrene does not start to solidify before reaching all of the parts of the mold.
This can be done in a home-made enclosure similar to a thick walled fish tank. Once the melted goo is poured into heated hopper over the heated mold....you slam on the lid and apply about 20+ inches of vacuum until goo appears at all of the vent ports in the mold. Then release vacuum slowly and use compressed air to cool the mold.
Trim the nubs and sprues...then separate the mold and trim the flashing and sprues. Its not as easy as it sounds on some molds.
The type of mold lubricant is critical to the finished clarity of the part. Ray