Another unusual technique is to find a streatchy material (headliner cloth works), streatch it over a form and brush on some fiberglass resin. Makes great speaker housings, stretch over a bucket, when it dries, cut a hole in it where the top of the bucket was. You can sand it smooth enough to paint, or cover with vinyl or carpet. You can get some wild shapes with this, reinforce the back with more fiberglass or expanding foam.
You may also want to think about leaving a door between the trunk and fender - than would be a great place to store extra oil, tools, etc.