I used to have the same bag also and loved it....the reason it won't work with a glide, or an ultra specifically, is it's designed to attach over a backrest. I used mine when I had a passenger backrest and a rack but sold it when I switched to an ultra from a custom.
2018 Road Glide Special. 12 inch 'Freedom' shield, Mustang seat, Harley tour pak, Advanblack lowers, Motolights, rear crash bars, Headset plug-in, Ciro footpegs, Black headlamp trim, mesh grill covers, Flash2open garage transmitter, buncha other stuff!!