Arca plate advice needed

[M_Six]

I picked up a gimbal head to use with my new 400mm f5.6. The Arca-type mounting plate has one screw and mounts fine to the tripod collar. But the camera/lens combo will turn on the plate despite having the screw as tight as I dare tighten it. I've seen other plates with two screws, but the tripod collar only has one screw hole. Sorry for being a dunce on this, but how do you secure an Arca plate to a tripod ring so it won't turn?

EDIT: I was just reading Bryan's Wimberley P20 plate review and he says it has some nubs to keep the plate from twisting. So now the question becomes which plate for the 400 f5.6?

EDIT-II: Looks like the P-20 fits the 400 5.6. On its way from B&H

(You've just witnessed the learning process in action.)