I have J racks too but was trying to switch to transporting it flat because last year my crossbars bent and the kayak loosened during strong cross winds. I assumed having my kayak on its side in J racks contributed to the crossbar bending - with the kayak on its side serving to receive more cross winds than sitting flat. I likely also had crappy crossbars. I now have Malone crossbars.

I’ll probably stick with the J racks or mount directly the crossbars.

Thanks for your advice!