I would go with a custom made box.. The Xbs have a ton of space in back behind the rear seat to fit just about any practical setup that you would want to put in there..
There are a TON of sites that will help you give you specs and give you a play by play on how to build it what type of wood to use etc.. I would pick out the woofers you want then do some research on the best specs for that specific woofer and then either build one or have one built for your specific setup.
I see you are located in the LA area. My fabricator lives in Westminister CA.. If you are interested in having him build you a custom box PM and I will give you his contact information.. He builds fully custom SEMA and CES vehicles every year..