Fiberglass bathtubs can be built from scratch using a pre-made bathtub mold. Bathtub molds come in a variety of shapes and sizes and can even be customized. Fiberglass molds are made using fiberglass gel coat and layers of fiberglass mat. The mat is saturated with a fiberglass resin mixed with catalyst to harden. Fiberglass is fairly cheap and can be found at your local marine supply store.
Buy a pre-made bathtub mold that fits your specific measurements.
Clean the entire surface of the mold using a clean rag and acetone.
Apply three to four coats of wax to the surface of the mold using a foam applicator pad. Let the wax dry and buff it out using an electric buffer.
Cut a piece of fiberglass mat the same size as the flat bottom of the tub. Then cut pieces that will cover the sides of the tub. Use these cut pieces of mat for a pattern on the rest of the layers of mat. Cut the number of layers that the tub mold manufacturer recommends.
Put on a respirator and apply a coat of gel coat using a gel coat spray gun. While the first coat is still wet, measure the thickness of the gel coat using a coating thickness gauge that measures in mils. The gel coat should be .20 mils thick when it's finished. If the first coat is not .20 mils thick, add another coat until it reaches .20 mils. This usually takes at least two coats. Let the gel coat harden.
Mix fiberglass resin and hardener together in a small bucket. Follow the recommendations on the containers for the correct amount of catalyst.
Wet out the surface of the mold with the resin mixture with a felt roller. Apply the first layer of mat to the tub, completely covering the mold with mat. Do not overlap any of the pieces of mat, butt the edges together. Wet out the layer of mat using the felt roller. When the entire layer is saturated, remove all of the air bubbles using an air roller. Repeat this process until all of the layers of mat are applied to the mold. Let the fiberglass harden.
Use a flat pry bar and slowly work the fiberglass tub free from the mold.
Flip the tub right side up and trim the rough edges of the tub mold using a right angle grinder with a diamond blade.