Hi, I had a very similar problem. It was not a large quilt (twin
size). Well, when I finished my attempt I was ready to throw it into the trash. I removed it from my sight for a couple of days...When I looked again it was still horrible. I was going to rip out but decided to put on another backing fabric. It took a long time to pin it on tight etc but I did. With right sides together I stitched 3 sides, turned it right side out and then closed the opening and top stitched the edges. I was prepared that it might be too loose and decided if that was the case, I was prepared to tie it only to the back. It finished perfect.
No tying necessary.
Good luck. Dorrie