We use good quality bungees.
We tried the screw on ladder clamps but in the cold or if if your hands slip, boy oh boy, do they hurt and they take so much time to undo.
Also, one day one was not secured properly to the underside of the roof rack and in undoing it, it fell onto the windscreen and smashed it.
Having said that last year I let go of a bungee a bit to quick and it smashed the rear window.
Cheers