I managed to find what I think is one of the old doors in the garage, but it's trashed and is missing the hardware. It wouldn't be impossible to modify the taller doors to fit; I might get the carpenter who eventually works on our catio to do it. But now that I think about it, the new carpet's probably covering the floor hardware.
It might be smarter to just get something new for downstairs. Finally, an excuse to check out the Habitat for Humanity store.