Buy a flat, seamless table that is at least 4 inches bigger than the panel on all four sides.
Spray the face of the table with a can of industrial strength spray glue. Before the glue dries, cover the surface of the table with a sheet of mylar plastic. Lay the mylar so it's flat and has no creases or bubbles. Let the glue dry.
Apply a layer of boat wax with the foam applicator. Let the wax dry, and wipe away the excess wax with a clean rag.
Cut the fiberglass mat 2 inches bigger than the finished panel's dimensions on all four sides using a razor knife. Cut the number of layers that the panel needs to reach the desired thickness.
Mix the fiberglass resin with the hardener following the recommendations on the containers. Use a small bucket and a stir stick and thoroughly mix the resin.
Wet the surface of the table with the resin mixture using a felt roller. Apply the first layer of mat to the table, and saturate the mat with the resin until the mat is clear. Remove all of the air bubbles from underneath the mat using an air roller. Continue this process for each layer until all the mat is applied to the panel. Let the fiberglass harden.
Pry the rough panel from the table using a flat pry bar, slowly working your way around the panel until it breaks free from the table.
Mark the panel's outside dimensions on the panel using a pencil, large square and a straight edge that's as long as the panel. Put the straight edge down so it's 1 inch from the edge of the panel and make a straight mark the length of the panel. Pull all of your measurements from this line. Use the square in all of the corners. Measure from corner to corner after your marks are complete to check for squareness. Both measurements should be the same if it's completely square.
Put on a respirator. Use a right angle grinder with a diamond blade to cut the panel out of the fiberglass.