In the pics below I’m using foam pipe insulation over my Thule square rack bars. Cut into sections, each section is secured to the bar with bungees. In the middle section, I left a one foot wide foam tube section to be removed using velcro strips. In the last two photos, you can see where I removed the section so I can add a Yakima or Thule kayak stacker. Using the stacker, I can attach 2-4 sea kayaks on their sides and several SUPs stacked on their deck or hulls on the other side.
Before each trip, I do a shake test with the rack bar before adding gear to make sure it’s 100% secure. On large loads I’ll throw two straps over the entire load and run them through the doors as a backup.
Note: Yakima kayak stackers do work on Thule bars if the plastic tabs are removed on the square tie-down sections. I add additional pipe insulation on the kayak stacker pipe for extra protection.