March 2025 update.
Per the GFC forum, it's not a good idea to run stronger gas struts to lift the roof panel the mounts can shear right off. So I've taken the ski box off.
So even though I posted all the how to stuff below, you probably shouldn't
you can read more here https://forum.gofastcampers.com/t/upgraded-gas-spring-for-higher-load-rating-on-the-tent/3522/17
August 31, 2022
Will need to add stronger lift struts, but I'm really happy with how this turned out.
Starting off, I painted the upper part of the Yakima Skybox with Aluminum Rustoleum paint. But it was way too shiny and showed any imperfection in the plastic. So I got the idea to thin some black gloss Rustoleum and spray it with a too large tip on my HVLP spray gun. I wanted a slight splatter look, and it looks kind of cool I think. Should be easy to touch up in with a black spray can in the future, when low hanging limbs scratch it up more.
In the slideshow you'll see the exact parts I used, but in summary, I found square ubolts that are meant for boat trailers that perfectly go around the GFC beef rack bars. The problem with GFC roof rack bars is the lack of clearance between the bars and the white roof panel, but the square ubolts slipped under fine.
I think I need to lengthen the slot opening a little, so I can move the box back just a few inches more, for aerodynamic purposes. Other than that and adding the strong gas lift struts, I'm happy with how this turned out. Gives me a secure place to add stuff and lock it out of sight.