Made my own under seat mount that allows me to easily remove the backrest when riding with a passenger. Then made a few minor changes to the backrest it's self so I can just slide into the mount.
I went to Home Depot and purchased 2' of metal as shown in the pic below. I measured from underneath the seat to the opening which was roughly 5 1/2 inches. I cut two piece of the metal and welded them together making a sleeve for the backrest arm to slide into. Took another flat piece of metal and used a hand grinder to cut it 4" in length and 1 1/2 wide as the arm for the backrest. It was necessary after drilling the mounting holes to bend this piece slightly back so the backrest stands up straight and allows a comfortable position against the back.
If you look at the previous pics you can see I used a piece of flat metal also as a way to attach it to the seat.
Rottie, the sleeve is permanently mounted on the cross bar I mounted under the seat. The Backrest arm just slides right into the sleeve and is a solid mount.