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You need the right kind of plastic for this. Some plastics are easy to shape but not easy to weld; others are easy to weld but difficult to shape. Obviously, you need to find one that can do both. Do you have experience with plastic welding?
Fibreglass is lighter & easier to use (if you don't mind being itchy as he// for the next few days) but is usually based on a mould.
Having said that, it is possible (not easy, but possible) to create the desired shape without a mould & join several pieces together