Hey..I have a 98 and am currently doing that. Figured the back is only comfertable for one person anyways. So I'm designing mine and will have it fiberglassed and painted white to match my dash. Here are pictures of it being biult. The skeleton if you will. I've added a few pieces since. I've actually biult a airway path and have siliconed all seems to get a good enclosure. I just need to layu the fiberglass down now. I've never biult a box before, and i've never fiberglassed before. This may be poorly designed or whatever. I don't care its been fun and a learning experience either way. Kind of a make or break deal lol. Heres my progress a few days ago.
I did have do some grinding and I've been careful to try and fill all holes in the floor of the truck. Afterall, its a woods truck, it'll hit water. So I wont be putting in the most expensinve sub. I'm going to fiberglass the top white and brown carpet the base to match the current carpet.