there aren't any out there that i'm aware of, but consider two things: XJ engine bays barely get enough heat out as it is, and that spare will severely reduce forward vision due to the XJ's relatively narrow windscreen (I know because I've tried this before :) ).

It may be possible to mount the spare carrier from a TJ on the hood/bonnet, but I'd be concerned about doing this on the relatively thin sheet metal.

Check out www.naxja.org for more ideas. You'll probably need to go with the roof, though.