Don't trust the arms for that luggage rack to hold much weight. The stock setup is rated at 3kg (6.6 pounds), and that ain't jack. Many have snapped them (yeah, me too.. but I was using it to hold a big flag mount. I buddy is fabbing up a much stronger one now.).
Here is what I used to add a tie down point for my tail bag:
Attached to the bottom of the passenger pegs with existing bolts.