The Bonneville flat spoilers also have vertical plates on the sides.
I took the 10" spoiler I mounted at 30* last week (had it on for a day, and it made things worse) and test-fit it without the 3" ductail spoiler in the way. It actually sits pretty flat, but there is a 1-2* upward angle from the 3/4" lip spoiler I've had on there for quite some time. My trunk does have a slight downward angle, though.