I finally found my way out of making the film holders so forget the filmholder prototype pictures. They only serve as an example on how not to do it.
Don't make things to complicated!

The rear box is now finished an oiled. The baseboard is ready but not yet equiped with locking knobs. I'll make the knobs for that later this week.
That is, if I find the time to do that. Our daughters' baptism is next sunday.