If you look at the ones in a new car they are not exactly substantial in girth. Some are probably 1/8" solid round stock. The key is how they are attached. On the bottom ones I would try to tie them into the existing seat belt bolt holes. The package tray was never designed to take that kind of load (in an actual accident) so I would get a lot more creative there.
Good idea for a thread, I've been thinking about this too. I just might not have mine done before car seats are no longer required anyway.
