Tuppence worth....
I use a bike carrier (made by Thule, i think).
It's got the simple loops that clamp around the wheels of the bike with quick and easy ratchet (spelling?) type fastenings that take a split second to open yet are very secure at speed, the other bit that clamps around the frame of the bike just sits flat and isn't needed... unless it's the weekend and my bike is up there!
It's cheap and purposeful - ok I admit it's not secure but who wants to pinch/walk off with a 6ft wfp in bright yellow?