Well the amount of airspace is specific to the subwoofers, mine ideally work in about .85 cubic feet of air per sub (in a sealed enclosure), if you have Kicker's than they will be different as I have a pair of Alpine's. Look online or in the manual for the subwoofers as far as the air space goes. If you don't need the seat you can fit a fairly large ported or sealed enclosure. If you buy the enclosure make sure you buy subs that will work with the airspace or vice versa. If you build it, what we did is find a box online that was made to go behind the seat and housed the correct air, and we used the dimensions and widened and shortened it so it looked a bit cleaner and then built it. We have a ton of 1/2 inch OSB board that we used to build it, normally 5/8, 3/4, or 1 inch OSB or MDF board is a bit better but if you seal it up good enough I don't think thicker wood really makes much if any difference with 10" or 12" subwoofers. I wouldn't say it's hard to build it if you have the right tools, and if you don't need the seat you can build a much easier square box which can allow more air space as well. Kind of a long message but if you have any other questions about building your own box or the correct airspace, etc I might be able to help you out.