I know it is not a funny subject, but maybe a hump back padding would help? Turtle shell?
I can't help but think that a big half round piece of foam attached to a belt, like half a life jacket would not only be funny, but it might actually work.
Seriously though, can you round the top half of your back? You know, the shoulder area.
Almost 80-90 percent of falls and rolls are done across the upper back, not the lower back, and with squeezing your shoulders to the center of your chest to make the upper body round. Across the shoulders at a lesser angle than right shoulder to left hip style if perfectly valid for ukemi.
If we can't get you to round your back, I will make you a foam turtle shell to give you some roundness. It's not fun to hurt yourself with practice, and falling/ rolling is a big part of the learning process.
Hope you find a way to make practice better.