Her are some pictures of the bracket.
It might not be finished yet.
I've shaded in the parts here becasue they were hard to see.
Its a piece of 3/4' square tubing with two tabs (flat bar) welded to it. In the middle of the 3/4" stock I drilled a hole put a 3/8 x 4" bolt through it and welded it so it wouldn't spin.
On top (above the rim, not shown) is another shorter piece of 3/4 square tubing with a hole in the middle and a nut welded to one side. It acts like a big wing nut.
I just set the bar on the roof, plop the tire over it and hold the bolt and tighten the "wing nut" until the tabs catch the bottom of the rail and stop trying to spin.
4" is really too short of a bolt and i have the tire mounted shiny side down. To mount it valve up (like to be able to check pressure easy) you'd need a much longer bolt.
Current design pretty much tells you when the pressure is down though because with no air in the tire the wing nut would come loose