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Looks like the OP has passenger pegs installed. Viking makes a swingarm bag for the SoftTail too (That's the one I have.) Due to the rigid plastic backer, it makes it relatively easy to install with the hard mount kit. No picnic, mind you, but not bad.
This can be mounted still giving access to passenger pegs. You still the mounting holes in the plastic backing of the bag wherever you need them to be, so you can mount it a little farther back on the bracket and keep access to your passenger peg