Rear wheel mount.
Search "boat freezer compressor" "galley fridge plates" custom fridges are more common on fishing boats and yachts, check boatersworld dot com and boating/yachting forums. Check Ebay for fridge plates and compressors too, most are probably too big for a very small container. Also check for RV fridge compressors and plates.
However, building for the back of a jeep isn't a very big project. You might be better off modifying an existing cooler putting plates inside and the compressor outside. In fact, sounds like a tiny project with the space you're filling as a fridge goes, you'd probably be better off going with a manufactured unit.
Another issue is the power requirement. Do you have a dual battery setup or a genset? I used a small bar fridge from WalMart with an inverter and separate battery for short outings in the Safari Van.
By the time you buy the parts to build a fridge you'll be close to buying a manufactured unit. The advantage of a custom fridge/freezer is fitting into odd spaces or custom spaces with extra insulation and building a top loader in larger spaces, 9, 12, 14, 18 cubic feet, 3 cubic feet isn't very cost efficient.
