thats a pretty easy design. you would need fiberglass, the hardener for the glass, wood, lots of high grade sandpaper and paint, wax (the wax you use on surfboards wax).
basicly the best way to go about it is to do it in your trunk. take everything out of your trunk (AND I MEAN EVERYTHING!) so just metal is there.
then you want to measure and sort of picture where u want this to go.
then ya got to make the mold. pretty simple to do is to use wood and cut out the holes for the speakers and build up the openings with small pieces of wood. next take the frame lay it inside the trunk and get the fittings right and make sure it looks the way u want it to. (you dont need to leave it in the trunk to do the rest)
now wax the living sh*t out of the wood so the fiberglass doesnt stick to it (if you dont it will be very hard to take off and u will probably break it.
next step is the fun part.. lay down the fiberglass so it covers everything dont worry if it isnt pefectly flat to the surface..
then lay the paste of hardener to the surface and smooth around get everything nice and covered then let that dry.
once dry cut ur holes for your speakers very carefully.id suggest using a drill bit on a high speed or if you dont got the size drill bit take a piece of the overlap and try different stuff until u find something that cuts very well.
then sand like youve never sanded before! making everything perfectly smooth is the key.
next step.. take the molding off the fiberglass and paint it whatever u want. make sure to use lots of clear coats though.
now with the wood mold make the rest of the box and seal it good.
next is put the speakers in the wooden box and screw them in.
next put the finished fiberglass over the speakers and screw in at sides (OR glue depending how well you are with screws)
and last but not least. carpet everything and glue edges to edges on fiberglass.
now you can place everything back into your trunk and place your new display in also. hope that helps.. i got most of that from the textbook that im reading for school, and the rest i talk to some pros from comp's.
BTW: if any trouble with any of this goto your local library and rent a book on fiberglass and speaker design.. it will help out greatly
Or if you have no surf shops around you can just lay a strong tape across the whole thing! Duct tape works REALLY well, and it'll peel off the back of the 'glass pretty easy. Oh and don't forget to use MANY layers of fiberglass matting for ultimate strength!