first decide what speakers you want to run. find out how much airspace and power they need. decide if you wanted ported or sealed box. I would avoid bandpass, they are tuned to a certain freq. and people are prone to overdrive this setup. experiment with box designs,and port lengths. This is the hard part, cutting. Once you get over the initial shock measure three or more times. Build your box, fit your speakers, then measure, measure, and measure again. Then cut your rear deck, I used a air powered body saw from harbor frieght (9.99) with coupon. lol... But it works quick and fast. try to make the cut and flow withit in one direction. If you cut right, the piece can be reused if you decide to sale.
its' not easy, but someone has to do it