I do not know if you have canti brake posts on your bike or not.
I recalled years ago seeing someone mention that he put that mini Nashbar front rack that mounts on canti brake posts on the rear to support a large saddle bag. I tried doing a search and could not find that post.
But I found this post which did not use the rack for that purpose, but he did mount that front rack on back.
https://www.bikeforums.net/17776623-post47.html
For aesthetics he had it mounted horizontal, which I think put it too close to the tire, but that was his call.
Or, another option. A friend of mine was looking for an idea to keep his large saddle bag off of a rear tire. I saw a DIY bracket on a bike that a bikepacker had made for his bikepacking style saddle bag and I took a photo for him, that photo is below. If I wanted to do something like this, I would have done it a little differently, but the idea is good.
