You can use insert type rollfilm backs. The Calumet C2 is the most inexpensive way to go. It's 6x7. Or you can look for the more expensive Sinar insert backs. They come in 6x7, 6x9 and multi forms. The multi version allows you to switch between 6X7, 6x9 and 6x12 in the middle of a roll. It's significantly more expensive than the dedicated formats in my experience. I have both a calumet and a sinar. The sinar certainly feels nicer and inspires more confidence than the calumet, but I can't say I've had any problem with the calumet at all.