Not easy. Just mounting it underneath would kill your ground clearance and could even have the spare tire hit on speed bumps. You would have to cut out the bottom of the trunk and weld in a flatter piece, as well as adding a pulley system or some other way to easily remove the tire on the side of the road.
I was considering that option, but you are correct Squishy. It's not easy, and there is too much involved. A lot of effort in re-improvising, plus having ground clearance reduced is a disadvantage. Fortunately, i have never had to use my spare tire. Anyway i prefer it in its current location, as under the cargo floor carpet.
Have found it easy , removing spare from current factory position. As you mentioned a pulley system would be required. It would be a pain. Crawl around under car, to winch down tire and winch up. I agree with you Squishy, as in it's not a practical set up.
You are better off considering the safest alternative...run flat tires. OR, you can do as Ford has chosen to do with many of their cars, install a can of fix o flat and a compressor...and a pat on the back.