I have your solution. I had the same problem with my HD CUSE backrest.
The reason only the top part of the backrest is hitting your back is a result of poor design. With the backrest installed, it has only one pivot point so moving it pivots the top forward cuz of the one pivot point at the base. This allows very poor or IMO no support of the back. My solution of five years ago was a Mustang (like Mustang seats) back rest, which I took off my old RK and put it on my 09 CUSE. It works great. The difference is the design allows for two pivot points, at the base and the top of the rest, which when the backrest moves forward, it moves vertically supporting your back as needed, not at a slant with the top leading.
That is the good news, a very well designed unit. My RK backrest has two buttons in the vinyl, matching my old RK seat. It does not look cool on my CUSE smooth seat with the grey stripe. It works great but I have to have mine recovered. If you buy a new one from Mustang, you can get it smooth black and you will be happy. I cannot believe HD is still marketing that useless backrest unit, but . . .they are.
Road Hog