I recently acquired a Pentax 600mm lens for my 645D. It quickly became apparent that more stabilization than the Acra-Swiss Z1 could supply was needed. I found a long lens support on Amazon made by Sunwayfoto. I took a chance and ordered that along with a rail and two clamps, allowing me to quickly adjust for other lenses. The items are extremely well made and appear to do a great job of stabilization. Note solid attachment at the lens foot and body along with support of the lens. Highly recommended.
_IGP1179 by tsjanik47, on Flickr