I have an idea in my head for a roof rack system where the bikes lean over diagonally as much as possible, with a strut coming up to grasp the top tube. The strut should be extendible/adjustable so you can lean the bike at multiple angles, depending on how many bikes and other roof cargo you're working with.
I don't have any good ideas how to DIY something like that.
(
Although many years ago I did this -- that was the system holding the mongoose that I drove into a balcony. I can't even remember whether that wooden mount survived the incident, but the rectangular cross-bars were bent)