Here is a thought.
You may need to try multiple womens bags at say an REI or a Backcountry gear store.
If you find a bag you like and fits but does not have the temp rating. Get some down pants and jacket- light weight of course.
I had a 1lb bag rated at 35 degrees.
Temps went below 20 . I have Montbell down pants and a good fleece shirt and I stayed warm with no issue.