I mounted my 48" Hi-Lift Jack behind the rear seat from rollbar to rollbar and used a cheap bike lock to keep it away from thieves.
To do this modification you will need the following:
1. 2-4 1/4x3" screws.
2. Corresponding 1" washers (or close too in size, just make the overall diameter fairly large, mine are approximately 1in.)
3. 2-4 1/4" bolts.
4. 2-4 1/4" wing-nuts.
5. 3' metal strap with holes, about 2mm thick.
(I wear a size 12 shoe.)
All of these were bought at my local Lowes (you could probably find them at Home Depot or any other hardware store) for under 15$. The only metalwork needed will be the process of sawing the metal strap in half:
1.Saw the metal rod in half, getting two 1 1/2' straps.
2.Use your knee to bend them circularly, in the shape of the roll-bar [Though you should form them to have the opposing ends join together parallel (see picture)].
3.Make sure you align the holes evenly. Insert the bolt on each bracket with the tip of the thread towards you (if you're standing behind your Jeep).
4.Tighten it down with the bolts. (Here you can insert two on each side for more stability, but I, personally, only used two for the whole she-bang and mine is sturdy.)
5.Now slide the jack horizontally over the holes, and tighten it down with wing nuts. When the weight of the jack is onto the bolts, the brackets themselves will angle in/downward, but the rod is strong enough to hold it, and the angle helps to keep it in place (think of the rope those guys use to climb the trees in Ultimate Outdoor Competitions).
6.OPTIONAL: I sprayed mine black to match after I took the pictures. Any other questions, ask.
Good luck, it's very sturdy and the bike lock keeps it from wandering eyes. Though, I have not yet been in extreme off-camber situations, in which the weight of the jack may be shifted higher, though I doubt it'd make a difference.