I love the Redrock Mattebox. I used it on three feature films so far and it is a good workhouse. One thing, however, I am not very fond of are the rubber donuts. There is a solution, although the solution costs a bit more than the Redrock Mattebox itself, which is to use the Arri bellows from the MB-20. Not only does this add a more user friendly bellows but it also adds 138mm filter support. I haven't done this but fellow Reduser, Damien Molineaux, kindly posted pics of his setup.