Thanks, @Jipe. Looks like I need to add some hardware and bushings to make it work.

On another note, since I'm starting with nothing and building to suit, what are the pros and cons of the suspended rack, where the luggage load is cushioned by the rear suspension, and a rack that's attached only to the rear swingarm? I'm particularly curious about how a 15 kg load affects ride and handling when suspended vs not. Thanks everyone for your help!