I have walled off vehicles before for just a single 12. If the OP wants to build a wall for 4 12s(which many people do) then why would any of us care what he does to his car?
With that said walling off a vehicle is not all that hard. It is time consuming as most the wall will be built inside the vehicle in most cases. If you do everything right in the design phase though it will become very easy to build. Just make sure you have that right and you are good to go. Tuning a wall for what you want is different then tuning a basic application as you would imagine. With a little searching you will see what the majority of builders do for tuning and how they come up with their ideas.
SMFJ- MDF is about $30-35/sheet and is about the cheapest your going to get. Depending on how you build the wall and bracing you will need other types of wood, various brackets, glue, and some sort of silicone or other sealant. You may also want to resign or even fiberglass the inside depending on the amount of work and what you want it to look/sound like. The front part of the wall which is the baffle is the most interesting and there are several ways to go about building it. Depending on your abilities and tools/materials you have handy you will have to choose which method is best for you.