you dont neeed all that. my 18 is only half powwered and because my box it built to the right specs it makes all the difference.
and mess around with switching the numbers around in the length, width and depth and you can make just one long one
go to re enclosure calculator it tells you how long to cut the wood and all that! its way easier then trying to design one because it makes it as you go
i have a 90 cadillac deville.
my box is 4.5 cubes on the inside and 6 on the outside tuned at 35 hertz! its a big car soo it needs a lot of bass to actualy feel it and that box makes all the diference. but if you use up as much of the sapce in your car as possible you can drop the hertz to like 30 if you get the box to 7 or 8 cubes
well it tells you all the specs after you type them in and you can mess around with it and make a long port the bottom or or a ong one across the baffle just mess around with the WxLxH
go to RE enclosure calculator on google you can customize your own box to the specs you want it at!
thats what i did for my re sx 18"! and try to find out the recomended specs for it so you can build your box around it