We just received our first batch of Canon 5D Mark II cameras in, and of course we had to kit them out with the Arri MMB-1 Mattebox. We love the mattebox and have put together a custom kit that makes the 5D Mark II and Canon L Series Primes fit just right. Check out the video showing this awesome combination, and see how we made everything work together.
Update: In the video I use a Tiffen 72-77mm filter step up ring on the Canon lens. This ring is normally used to put a 77mm screw on filter on a 72mm lens, but in this kit we are using it as a step-up ring. The outer diameter of the Tiffen ring is 80mm, so it fits perfectly with the Arri step-down rings.