The Nikon Coolscan series of film scanners largely live up to their reputation, but thanks to that reputation there comes crazy costs with the film holders. When I first started shooting medium format film, I started with a 6×4.5 camera and the way those strips cut and laid on the FH-869S holder worked just fine.
When I added 6×7 shots to the process, things sort of fell apart. Since only two frames fit on a cut holder, the edge of the second frame would curl inward, and make a nice crispy scan impossible. The obvious answer online is the Nikon FH-869G holder, but with some online prices being more than what I paid for the scanner, I have not been able to convince myself to buy one… Add in that with that holder there are now 4 additional surfaces to keep dust free, and it’s just not perfect.
3D printing to the rescue! I had several iterations before one worked, but I now have a holder that holds all of the negative around 2 6×7 images out of a Mamiya camera (Mamiya has 6.5mm spacing between frames, other manufacturers may vary)
The holder comes in two pieces, one as the carrier that goes into the scanner, and the other is a mask that ‘snaps’ on top with magnets and keeps everything flat.
After the magnets are added, I also recommend adding some flocking paper/tape on top of the two surfaces that touch the film to keep things nice and soft. I bought these two things from Amazon and used some cyanoacrylate glue to hold the magnets.
Flocking Tape: https://www.amazon.com/dp/B087PDRZKT
Printable files were added to Thingiverse here. Now that I have a working outline and mechanism for the holder, I intend to experiment with some other ideas as well.
A word of warning… This is not a small print, and has several critical dimensions/flatnesses. Please measure and compare to other holders once a print is done. Anything you put in your scanner is at your own risk.