That's what the Koss is, but it has the 1/8" plug. Bear in mind the 1/8" jack on the box is not an output, but an input. I was thinking the problem was the volume going into the Canon. There's no "headphone hack" for monitoring from the box. You could split the output from the box before going to the Canon, but that doesn't seem to be what the OP was talking about. Since he mentioned "tape", I was assuming the issue is overdriving the Canon. If the camera guy is listening from a split and that's the problem, then, yeah, there are lots of headphones with volume controls.
I've worked with the 5D MkII a lot and the last thing I would want to deal with is another XLR cable and another little box to rig into the system. It's hard enough worrying about keeping your balance while walking and shooting with an EF 70-200mm f/2.8L.