Sounds like fun. I always enjoy tweaking with car electronics, except for removing door panels - not a huge fan of that.
I know there are some very cool behind-the-back-seat DIY sub setups for my Sport Trac (I got a special edition of it with a factory 500W Pioneer system with a decent sub that I am very happy with after I changed the head unit and put in a rear view camera, so I haven't done anything sub-wise), so I am sure there are some for the Ranger, because there actually isn't too much difference between the two. I get all my information on mysporttrac.com, which is a forum dedicated to the thing. Honestly, those forums may be sufficient to find a decent box build for a Ranger, but if not, I am certain that there is a similar forum for Rangers, which has similarly excellent information on custom sub setups.