Some readers have asked for better pictures of the film holder I am using, so here we go.
I hope this attempt to explain the structure is more successful than the photo in my last post.
The body of the film holder is made from two stainless steel rulers. I carved a ~0.3 mm deep and 0.5 mm wide “stair” to the edge of the rulers (using a Dremel miniature drill mounted into a router table). The film lays on top of the stair. Then I glued four plastic strips on top of the rulers (the blue ones in the picture): two to keep the rulers tugether at the correct distance and two to hold the film flat from its edges.
Here is a side picture that shows the structure in more detail:
The design has worked quite well - the film still bends a little but not too much (the rotation needed for frames in the same scanned strip is typically within 1 degree). One design flaw in this setup is that it is not possible to remove the film in the middle of scanning a rollm but in practice this hasn’t been too big problem.