I still wasn't happy with the design from my posts above. I did a bunch of searching online and found this one I really liked.
Then, I finally started cutting and welding this week/weekend. I haven't welded since I was in high school, so I wasted at least 5-6 hours practicing my skills. These pictures were taken earlier today and I've done a bit more assembly since then.
The first picture is the top and bottom. I was trying to figure out the height. My magnets are 4" and I ended up going with 5" peices, so it sits a little higher/deeper. From the bottom of the cargo area, to the top of the top rail is going to be about 6".
The second picture is the bottom of the rack sitting in position on the roof.