Here's an old Photo.net thread on adapters for the camera. I believe the adapter you need is designated as an "M" adapter for the Mamiya Press camera. A company named Four Designs sold these years ago and allowed 6x7 and I think, 6x9 Mamiya backs to fit the Polaroid 600SE. Probably hard to find now, but I picked one up from KEH many years ago so you might try there. Midwest Photo Exchange might be another place to check.

There's a slightly different model than the one I have currently on ebay. Mine is a horizontal format and the mounting tabs have a slot that mates with the 600 SE. Based on the Photo.net thread, you would need to make a slot in these tabs to use this style adapter. (Perhaps all Four Designs did was machine a slot in standard holders available at the time.)

Joe