if you have 2 roof rack bars, put them close together, buy a decent thickness steel plate, drill holes to mount with u-bolts on the factory bars, drill hole in the center with a a proper length bolt to tie down the tire with.
its just an idea...not even sure how safe it would be, but it'll be cheap.
That is only for a wrangler as you can see by the picture, and what was stated earlier, the thin metal cannot handle the weight of a tire. if you are some what inclined with tools and are willing to put in some work, you can head over to your local JK and find a blazer or something like that and use it's spare tire carrier. you will have to reinforce the rear quarter panel and mod your rear bumper for it, but there are threads on here for a homebrew tire carrier made with these parts.