This is one piece that you can conceivably make yourself, although there are manufacturers who make them in a variety of styles. After cutting the strap out of the car, you can seal the frayed edges with a lighter, and then sew the ends together using heavy-duty nylon thread. The connecting belt pieces of the seatbelt can then be used as a belt buckle [source: Instructables].
Feel like being stylish and eco-friendly? You can make yourself a pair of sandals to match that belt. The same seatbelt you used to create the belt can be cut up and sewn to the soles of old flip-flops (with the previous straps removed) to create sandals. The material is strong enough to stand up to almost any foot traffic, considering it was designed to stand up to car traffic.