You’ve got a pretty good first prototype and now it’s time to use your experience with it as feedback to improve your next prototype!
I like how you’re thinking about neatly storing this type of resistance band since both types pictured in this thread can look pretty messy when lying around not in use. Maybe explore the storage theme a little more while incorporating a grip that would be easier for an older person to hold on to during exercising instead struggling to hold on to a thin sheet of rubber like FH13 mentioned. I’m thinking you could bond two cylindrical foam handles to the ends of the resistance band then roll them up on a flat surface into something kind of like a scroll. Just an idea.
I’m looking forward to seeing your progress and what you come up with. Remember, keep your prototypes simple so you don’t favor ones that took more work to finish and keep going until you find the best solution. Good luck!