That doesn't look bad at all Wally. Good on you for making something work.
Nearly 30 years ago I made a 1-piece discount store seat cover work on my 55-1st seat. Like you I used the string ties to attach the upper section to the back cushion. I cut some slits for the hooks that go over the top rail (pipe) of the seat frame. I put the upper cushion in place and attached the bottom section of the cover to the bottom cushion with the attached ties. Then I took a length of clothesline rope and stretched it taut in the crack at the rear between the back and bottom cushions, and tied off each end. That held the cover in place. I could tilt the back cushion forward, but not remove it. Less than ideal, but it worked, and didn't look too bad.
