When wheeling, mine is on the factory roof rack held in place by two pieces of 1/2 inch threaded rod sandwiched on the roof rack crossbar by 2 big nuts with nylon locking inserts. The jack is held in place with some big wingnuts and on one end a clip, the other end a small padlock.
Normal commuting has the jack resting from rear door to door up against the sheetmetal below the rear seat. It doesn't bother the occasional passenger and it's dry, quiet and virtually out of sight. I ran several eyebolts through the sheet metal there, one on each end and also on each side of the driveline hump. It works well with some simple bungee cords. Be careful when drilling the holes, there is some wiring under the carpet on the drivers side, especially on early ABS models.
I used to have it in the back from wheelwell to wheelwell, but the larger spare tire doesn't fit when it's in that position.