As I learned here (by reading other Threads), the TM spare tire carrier causes "deep impressions" in the sidewall of the spare tire over time. The entire weight of the tire and wheel is supported only by the 4 small points of contact at which the tire crosses the two supporting tubes.
Seeing this damage already occurring in my own spare, I went to a steel fabrication place and had them cut two plates of 1/8" steel, each 8" by 18". (If I had another chance, I'd make them even 19" or 20" long, but 18" really is good enough.)
The plates rest on top of the supporting tubes. The first photo shows a plate (painted), with holes drilled close to one side (look below the hardware).
These holes should be near one side, and NOT in the middle.
- - - - -
Second photo shows the assembly, with both plates bolted in place. Because the tire weights a lot and squeezes down (hard) on the plates, I feel that only one hanger is needed for each plate-- on the tube near the front end of the TM. Just one is sufficient to assure no movement after the tire is loaded on top.
I used hangers, instead of welding because (1) it allows for adjustment after seeing where the tire lies; (2) the metal tube is awfully thin; and (3) hangers with nuts and stabilizer bars, plus lockwashers, costs next to nothing.
- - - - -
Third photo shows the assembly after putting on the tire. This is upside down (I flipped it over to show the UNDERSIDE to the camera). The "circle" at which the tire is widest now rides on the plates for many, many inches (instead of just the four tiny contact points which occured before adding the plates). The 1/8" steel is strong enough to have almost no flex, but also thin enough to avoid consuming too much of the distance below the TM floor. My spare has always ridden tight against the TM, but this 1/8" makes almost no difference at all.
You do need to place the plates so that the hanger bolts and nuts are in free space under the rim, not hitting tire rubber. That's why I drilled the holes in a "mirrored" fashion, close to the edge of the plate. In picture #3, you can see that the plates are positioned so that the bolts are under the wheel, and not the tire.