Purchase a shower base mold online; one good source ist he Gruber-Systems website.
Prepare the surface of the mold by applying two coats of mold wax and let it dry. Wipe away any excess wax with a clean rag.
Put on rubber gloves and mix the gel coat with the catalyst in a small bucket following the recommendations on the containers. Using a stir stick thoroughly mix the catalyst into the gel coat. Add the gel coat to the spray gun.
Apply the first coat as evenly as possible in a heavy mist. Let the gel coat harden. Apply a second heavier coat to the surface, which will solidly cover the mold. Let the gel coat harden for at least one hour.
Cut a layer of fiberglass mat that covers the entire mold using a razor knife. Using the first layer as a pattern, cut four additional layers.
Add the catalyst to a small bucket of resin following the recommendations on the containers. Mix the catalyst into the resin using a stir stick.
Wet the entire surface of the mold with the resin mixture using a inch felt roller. Apply the first layer of mat without letting it overlap anywhere and saturate it with resin using the felt roller. Remove any air bubbles trapped underneath the mat by rolling out the entire layer with an air roller. Repeat this process for each layer until they're all applied to the mold. Let the fiberglass cure until it's hard and cool to the touch.
Grind the rough edges of the mold smooth using an air grinder. Insert the metal scraper in between the fiberglass and the mold and slowly work the base free from the mold.