I am selling the great Swingarm bag from my Super Glide Custom with the Dyna mounting plate for $150.00 INCLUDING SHIPPING to the lower 48. The manufacturer sells the mounting bracket and hardware for 1986-2017 Softail for $79.95. They sell this bag with hardware for $298.00. I am also listing it on the Dyna classified site. This is a very robust bag with a sturdy mounting system. I would try modifying the Dyna bracket to your Softail before buying a new one.
Here is the link to there website https://www.legendarycycles.com/