The T-Bag people make a "Universal" fit bag which uses straps with velcro closers to mount the bag to the backrest - this should work with the Ultimate backrest. Or, you can do what I did with my T-Bag when I switched from my Kawi Nomad to my '02 VTX Retro with a Hondaline backrest. I found a leather shop in my town who modified the pocket on my original T-Bag so that it would fit the new Hondaline backrest. Works fine.
...VTX-AL