Sorry for the delay in my response. So my options are to either find some old Graflex holders or find some Graphmatic holders. What I would really like to do is use a 120 roll back though. My main concern is that one of the slots doesn't have unlimited space. These pictures should explain it better.

This is the room I have to fit a roll back. I guess I could make the hole bigger though. It's about 7/8th of an inch by a little more than 3 inches.


This is what the back looks like:


Here are the holders I have, they are for film packs though so I obviously can't use them. Notice the grooves: