I swore I would never do it again by myself. An extra pair of hands would have helped a lot, but there weren't any available. Last night I went to Harbor Freight & bought a 1 1/2 ton (cheap POS) scissor jack. It has a rather strange top piece of metal, which is folded & must fit something, but I have no idea what it is (a body seam under a car maybe?). The top looks like this.
I had a small circle of 1/2" birch left over from cutting some trim rings for the port holes & one of them just fits inside the 13" the rim (the plywood disc is about 11" in diameter). I started doodling & came up with a simple metal bar with two #10 screws in it, which can slide into the top of the jack. This evening I went out to the shop & made the plate, no 1/4" stock around so I used 2 pieces of 7/8" X 1/8" X 2" long flat steel. Drilled & tapped them for screws, routed a slot in the bottom of the plywood disc & made a plate which will attach to the jack with 2 slotted screws. Put 2 nuts & loc-tite under the plate so it wouldn't come loose & they also act as a stop. It looks like this in the plywood on the bottom side.
Now all I need to do is slide the disc (with the bar on it) onto the top slot of the jack, tighten the 2 screws & the jack will raise the spare tire just by turning the handle on the jack. It looks like this when it's in place & fastened on the top of the jack. The top rivet on the jack acts like a lazy susan to turn the tire into position.
I still need to drill out the plywood with a hole saw for access to the the nuts & bolts, but that's another day. Next time I have to put that bloody spare back up, I'll just use the jack to raise it & line up the bolts, then screw on the nuts & be finished. It should take all of 3 minutes start to finish, with this new setup. Simple, cheap ($20) & hopefully effective as well. One less thing to worry about down the road.