How do I install brackets to roof so I can tie down stuff?

I was wondering about plastic stick-on hooks. Maybe reinforced with Eternabond?

Spring clips sound good, but I don't know of any that wouldn't pop off that skinny little gutter in the slightest PUFF of wind. There is just not much for hem to grab on to.

I am interested in this question for my new van. My old one has a roof rack and I clip to that. But the new one will not. I need an awning so rarely -- maybe 3 days a year -- that I don't want to invest in a real one. A tarp is fine, if I can figure out how to attach it.