And a right angle or adjustable jig. Cut the plastic sheet to the size and shape you need for your design. If you are using a CAD/CAM method then use a laser cutter. But you can also use a jig saw ...

And for finishing you'll need a mould cutter or jig saw, as well as a sanding board ... the way around and that the clamp presses the plastic sheet evenly on to the seal. Once that's in place ...