Just finished the rack and field-tested it this week on a 4-day camp trip in the Sierras. It passed with flying colors.
Built almost entirely of 1" square steel tube, the completed rack (without lights, spare, or jack) weighs right at 40 lbs. Its VERY heavy duty. I didn't want a cheep-o Chinese mail-order spot-welded flex-o-rack, so I built this. Plenty of room and more than strong enough for everything I carry-- a spare tire, jack, fishing poles, tackle box, rope, tow strap, etc. I came up with my own mounting system because I didn't like the idea of simply strapping it to the factory rack. My way eliminates the factory rack altogether and uses 6 of the mounting holes. The hole drilled in the mounting flange fits perfectly over the exposed roof bolt insert, which prevents the rack from moving. Its secured into the inserts with 6 bolts and lock washers and a scosh of red Locktite. The jack mounts to the side of the rack on 3/8" bolts welded to the side rail, spaced to line up with the holes in the jack shaft and secured with Ny-Lock nuts and washers ground flat on one edge. Its even padlocked for security.
On the opposite side from the jack are two "L" brackets welded on for fishing poles, shovel, or whatever I might want to carry. The spare sits on an X-brace and is secured using a length of steel tube welded to the center of the brace. I welded a threaded end onto the tube that uses the factory spare tire cone and wingnut. Up front are four mounting tabs for off-road lights, I chose to use Harbor Freight lights at $22 for all 4.
The floor of the cargo area is steel mesh, welded in every 2" all the way around the perimiter. This rack was built to carry a 31" tire but I could make one to suit any tire size.
To do again theres a few things I might do differently. The top rail should be round tube instead of square, much better for tying off rope or to use bungy cords or ratchet straps. Also, I'd make the spare tire hold-down rod much shorter so that the spare would be carried face-down. This would keep the wheel out of direct rooftop exposure to sunlight and would create another area inside the wheel itself to stow more cargo, perhaps a tool kit or a few spare engine parts. A small cargo net would hold it in. If I make more racks they will have these changes and perhaps a couple other small alterations as well.
Anyhow, lemme know what you think. I'd be happy to make one for a fellow Jeeper (for a nominal fee, of course.) Everything you see can be changed to suit any particular need.