I have just over 3500 miles on mine, fully loaded with a roof top tent and two awnings. Nothing has loosened. I don't carry a boat but why not just remove the rear crossbar, slide a piece of black pvc pipe over it and reinstall? Or, you could use the t-slot to mount a boat trailer roller with a couple of "L" brackets.
That's even a better idea than the fancy slotted thingofmajig! It can be made to be super tight and the round shape really helps loading by reducing contact area/friction!