i've been looking at these as well. i think i may just go with a full length roof rack.
do these flop around or is there a way to tighten them down at all?
To keep my drawbars etc. from making noise and to get rid of any slop, I replace the hitch pins with a 4 1/2" Grade 8 bolt with locking nut and torque it down to 100-125 lbs. With normal clearances, it should tighten enough to take the slack out and with it, the noise.
Good luck.
Al