My advice to you would be simply build it in ur trunk. thats about all u can do if ur gonna make it one big piece
Try building a model out of cardboard that will resemble your final idea and test fit w/ that. That's what I do when making my boxes to ensure that they fit into trunks properly. Also check to see if that back seat comes out. That can often be a larger and easier to pull in/out of the trunk. And if that doesn't work then I'm gonna have to go w/ jesse on the idea of one large box built in the trunk. Piecing a fg enclosure together in a trunk while keeping it properly sealed and looking good will be dificult, very difficult
im just finishing up a trunk setup in a corvette, and i found it easiest to make it mostly out of MDF. you'll definately end up with at least one seam in your design (i had 3 considering there are 3 amp racks), but another things that helps is iff you use a cover for your trunk buildup. what i did was built a false floor with some glass trim that went up to the trunk trim( it's a corvette, so the hatch made for a decent amount of open space between the weather stripping and the straight edged MDF). It looks alright right now (hasnt been sueded yet so we'll see how it looks finished later), but if you can get in there and make a good single piece Fiberglass trunk covering (in other words if you can make a single piece and still be able to pull it out of the car when it's done) then i'd say to do that.
-------------
2000 Chevrolet S10
Premier DEH-P860MP
JL Audio XR650-CSi components
Boston 4x6 splits
JL Audio 10W3v2
Alpine MRP-M350
Clarion APA-4162
do the back seats fold down? What type piece are you looking to make? And don't forget, you can always take the weatherstripping off for a few more mm.