$10 fix
I am running a 31" spare on the stock rim. I went to the hardware store and bought a piece of square tubing about 3-5 inches longer that the tire is tall. Then got 2-5 inch bolts, and 2 wing nuts. I also got 2 rubber caps for the ends of the square tubing.
I took it home and measured out the exact center of the square tube, then measured where two of the lug nut stud holes are on the rim and drilled 2 holes to shove the 5 inch bolts through. Then I JB welded them in place. So now, I essentially have a steel bar with 2 bolts sticking though it centered from end to end.
I painted it with hurculiner, just for the hell of it, and put the rubber end caps on it.
When I want to mount the tire, I put my homemade contraption UNDER the roof rack rails that run from left to right and make sure the rails are positioned about 25 inches apart. I put the tire OVER the 5 inch bolts, to form a "sandwich." The bar on the bottom, the tire on the top, and the roof rack cross rails in the middle. Then tighten the wing nuts down and it isn't going anywhere!
I even went as far as to get two rubber spring spacers to place between the roof and the bottom of my square tube, so that it won't bounce up and down on the paint. Granted my Jeep is an 85, and no one sees the roof anyway, but I like the rubber spacers.