I've only had the roll film holder for a couple of weeks so I haven't done either of the good suggestions regarding marking the ground glass. I should - my otherwise perfect composition is compromised by trying to judge where the 6X9 area is.

Good luck with the procurement of a film holder. As a number of folks have said, you can get 3X4 cut film and holders for the camera. I've shot a few hundred that way.

-Allan