There have been various 650XT models across various years and the rear carrier (rack) has changed over the years.
Which year model do you have? and what rack is currently fitted to the bike?
Even the Suzuki standard box requires a adaptor plate to make it fit the original rack, there is no direct clip-on option to the original rack.
Givi, Shad, SW Motech, Hepco & Becker, will all do an adaptor kit that will be specific fitment to the original rack - the parts kit you need will depend on the year and model of the bike.
You can use a strapped tail pack, various designs exist but they will all have straps you'll have to put around secure points of the bike, and generally I don't like them on the rack, I do use one on the back seat when camping.