If anyone here is curious about fan shrouds simply ask the vendor how they were made. Original shrouds were ALL made using the plastic injection molding process which involves running a large molding machine and metal injection mold tooling, the parts are made of thermoplastic polyethylene resin.

All of the reproductions I've seen were CAST in a soft silicone mold and were made from poured liquid urethane casting resin which is poured into the soft mold and left while it cures solid.

The reproductions look virtually identical to an original but will usually have some small dimensional shrinkage and will have ENTIRELY different material properties in terms of heat resistance, warpage, etc.