Tripod creep has been a problem forever. The cork or rubber friction material never seemed to do its job with even a moderate telephoto. It is easily but somewhat expensively solved with an "L" plate from Really Right Stuff or Kirk Enterprises. These plates are machined to wrap around the bottom of the camera preventing any rotation. My plate goes on the day I get the camera and pretty much never comes off. The added benefit is that it keeps the camera centered over the tripod instead of cantilevering it out to the side when switching to a vertical orientation. This is a much more stable position.
An even better option - were it available or even affordable - would be a full-frame digital Hassy 500CM, that square being both a beautiful shape in its own right, as well as a most convenient format for cropping to suit!