2.35 crop cover for Canon 5d?

[heyhey]

So a bit of a strange question here... I'm wanting to shoot 2.35 aspect ratio on the Mark III. This is a borrowed camera so I can't install Magic Lantern. I'm wanting to have cropmarks over the viewfinder (magic lantern would indeed be helpful be alas no luck) - has anyone ever created a manual 2.35 black cutout to place over the viewfinder or created a covering for one of these? :  http://www.adorama.com/LCDVF3C.html?gclid=CNT1ns-b4LgCFSU6Qgodyy0AVA  Or loaded a reference pic in the camera in order to get the cropmarks accurate?  If anyone has any ideas it'd be a real huge help.  thanks!