The gears were turning last night!
If you are building a custom frame you could include four 2" receivers at strategic locations on the sides of the frame, and build matching hitch mount spare holders to carry the tires. Sure this will widen the trailer, but unless you are building a 5 wide, it still shouldn't be much wider than the Jeep.
Put 1 pair on each side after the axle and the second pair before the axle, that would balance the weight, however you will have to remove at least one of the front holders to access the inside of the cabin, but at camp this shouldn't be a problem, as all the street tires can be stacked and locked for security.
When the extra tires aren't needed leave the carriers at home and you still have a handy place to mount accessories at camp.