Make them from MDF. Thats the common material if making yourself. Combinations of 6mm and 12mm. What you normally do is trace round the outside of your new speaker in 6mm mdf. That gives you the size and mounting holes for it, then look at the then do a similar thing for the holes that are on the door panel. Glue them both together. Paint with backboard paint or spray to make them look nice and waterproof. Done ;) Alternatively go to your local car hifi place and i am sure they can help