If you can find one, and its a big IF, the folding front seat section from an old Willys Wagon can be modified to work as a single rear seat for a CJ, YJ or TJ/LJ. It can be folded forward when not in use. There is enough room along side for a cooler/fridge/etc. when it is in use.
Here is one in a photo harvested from the Internet:
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I have a Willys Wagon front seat section that I scored many years ago that sits on a shelf in the garage gathering dust, but I can never part with it because I might want it for for the CJ-2 or CJ-3 I might get someday to match my Bantam trailer.
Here is my Willys seat section gathering dust:
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