Thanks for this correction @windguy! I couldn't find them online, but I saw the Yakima part number in your post, which was very helpful. However, I only found them for $500-$600, which still feels a bit pricey.
I followed @mrtn's link and found these for $200 + free shipping (impressive, given they're based in Germany)! They're rated for loads up to 75kg, which is more than enough for paddleboards or a skybox. I think I'm gonna go with this option. Wish me luck!