2008 f 150 bass options

I recently got an '08 Ford F150 Extended Cab. It does not have the factory subwoofer. I am looking to upgrade the sound system for a strictly SQ system. This is going to be a somewhat budget system, as I don't have a lot of money right now. I have everything else figured out, except for the location of a subwoofer. This is primarily a work truck, and I can't afford to lose any space. I have seen MTX's thunderform and was impressed at the location of it (under the rear seat), just not the price. I am not a carpenter, but if I take my time, I can put together a fairly complex box if needed, but simpler is obviously better. I have taken some measurements, but I don't have them with me now. In the meantime, does anyone have a truck similar (2004-2008 F-150 are the same body I believe) or have seen something that could give me an idea? Finally, I haven't been into the mobile audio scene for some time now, and I have no idea what options there are for 8" or 10" (I doubt 12" will fit) woofers that do well in very small spaces, can anyone give me some suggestions? Thanks for your help everyone...