How do you fix a large hole in plastic?

You can patch the hole with epoxy and fiberglass. Start by cutting two fiberglass patches 6 inches (15 cm) larger than the hole. Then, mix even amounts of each part of the epoxy in a bucket using a stir stick. It should be very thick and tacky. Use a putty knife to spread a thin layer of epoxy on one side of the hole. It should be thick enough that the fiberglass can stick to it, but not overly thick. Press one of the fiberglass patches onto the epoxy, and then paint another layer of the epoxy over the fiberglass. Let it dry for 24 hours so it hardens and cures. Then, repeat the process on the other side of the hole. Once both sides have dried, you can sand the epoxy smooth and paint over it if you like.