I know I'm bumpin this up from the dead, but - Would it be safe to assume that their matte box that requires rail support (The CineCity*::*DSLR kits*::*Matte box Mattebox Sunshade universal rod for DSLR 7d 5d MarkII camera), would hook up to RedRocks shoulder mount/railing platform? The rods seem to be the same size in diameter, but the spacing I don't know about...

Their matte box looks to do what I need @ a great price, though I'd like to use redrocks shoulder mount for its feel/design.

Actually, CineCity also makes a similar shoulder mount (if not the same) as the Redrock... and for quite a bit less, (only difference is that the wing-nut is red instead of blue). However, a shoulder mount like this is still an overkill for the 5D -- the camera is already so front heavy that there's no need for a padded cast iron shoulder mount (that also makes the rig even more heavy). Hence, I sold mine and made a simple & light carbon fiber shoulder mount instead, which also works much better:

-- peer