There are many ways of going about this... here are a few I have seen/heard of so far:
- JB welding a nut to a plate
- Using a square-holed square washer with a carrier bolt
- Bolting through the entire rack
- Screwing into the roof rack mounting holes
- Using a wood screw into the roof rack
- Making your own mount
Lots of options...
So even if I listed your idea here, please explain your method fully and provide pictures/etc.
Let us know if it's worked for you and what you would do differently if done again.
Any pictures of long-term results?
I'll chime in with my own ideas/experiences in a bit...