If you're still using the OEM I/S seat, look underneath the back end...the bracket unbolts. I believe you should be able to fabricate a small piece to bolt on there with one hole in it to match the location of the bolt where the two fender halves come together. That's how the Std. and Tourer pillions are held in place at the rear, that single bolt where the two halves join. Would be easier than fabbing up a cross piece.
Soo. I got some 304 SS 1/2” square tube
Welded in a M6 connector nut ( long nut ) to each end inserted and welded
Pop riveted on a little Teflon strip to the under side
I cant seem to load a photo
But now that it is polished it looks like part of the sissy bar rack 1