If I were wanting to play around with a 2-1 perspective ratio considering what you have on hand and not wanting to buy anything new. I would use 4X5 sheet film and would take a damaged dark slide and cut it to the width that would give me a width equal to one half the length of a sheet of film (appr. 2 /12 inches). I would then use this cut down dark slide as a mask that I would insert into the holder after I removed the darkslide prior to exposure. To give me an idea of how the negative will appear compositionally, I would mark off my ground glass with litho tape to show the width as it will be exposed. I wouldn't worry about the unused and unexposed section of the sheet of 4X5 film. Considering the hassle of taping 120 film into a holder, the waste will be relatively inexpensive.