I would use fiberglass
I have made various fairings for aircraft by making a plaster mold from a part. If your shower pan will hold together well enough, I would grease it up good and build a plaster mold on it. I embed strips of wood and/or metal in the molds to make then strong enough to handle. After removing the original, you can do some detail work on places where there were cracks or holes in the original.
Many cities have firms that refinish old bathroom fixtures. One of these outfits would do a fine job of putting on a gleaming finish. Or you could just give it a couple of coats of enamel.
John W. Irwin
2005 Classic 28 "Sabre-Dog III"
2013 Silverado 2500HD Duramax/Allison LTZ
WBCCI #9632, TAC TX-10