They start out by coating the onderside where the spare tire would normally go with vaseline, or any other releasing agent. After that you use fiberglass ribbons, that when saturated can be laid over the floor boards making the perfect fit. You can then use a template to cut a piece of fiber board out that match's the carpet on the floor of the trunk to be secured to the top of the fiberglass mold. Then you simply cut the holes for the sub, or subs in the fiberboard, and secure them in place. If there is going to be more than one sub, you have to create a dividing partiton between the two seperate sub enclosures. These types of boxes, or custom installations generally will not have anywhere near the sound quality of a regular wooden box for the sole fact that the amount of airspce that the speaker was designed for is very hard to judge when making a box that has virtually no flat sides, and is irregular in shape. It also severely limits the amount and type of objects that you can put in the trunk, and the proccess is expensive and time consuming. If you need any more info feel free to ask. There are a lot of smaller details in it that I left out.