This is what I'm using as a basis for my design of the light traps. These digital photographs are of a 5x7 wooden holder that I had sitting around and it has one piece of brass with a finger / comb piece on top of it and all covered with some felt like material.
I'm going to use one piece of brass shim stock covered with velvet like the wood holder shows in configuration. The brass wraps around the holder and is continuous from one side to the other. There is a wood piece on top of it where the locking screws are attached into. So there are two pieces of wood. One has an opening for the septum plus film and has a surface that is even to the dark slide plane; it has a recesses on both sides for the velvet light trap to go into when the slide is inserted. The top piece is thicker at the bottom where it sits on the light trap. The trap brass/velvet is not glued to the holder and stays in place because it is wrapped over the wood and held down by the top piece and a cover is place over it.
The old manufacturers, designer, inventors spent a lot of time work to make this design. I have a holder made, it's made of mahogany pieces that I had in the shop. I've made my septum groove 1/8" and the dark slide 1/16" but I'm going to make a septum groove 1/16" on the first 'real' holder to exact size.
My first model is a size of 5x9 inches, there isn't a film size for it, it's to see if I could actually get the holders made and to experiment on the assembly and procedure before I make a set of them.
I found that Garolite XX cuts very smooth on my Powermatic 66 cabinet saw with a fine tooth carbide blade, no problems with that. I bought a 1/16" four wing carbide shaper cutter for my Powermatic 27 shaper. It's like cutting through butter. I got some new DeStaCo clamps for a custom cutting platform jig.
I have an excellent planer and a Uniplaner so surfacing the stock isn't a problem. What I thought was hard is easier and what I thought was going to be easy is a bit harder.
This week I'm going over to get the cherry stock that I'm going use for the holders. My plan is to make 6, then make a second run of 6 for a total of 12 11x14 holders. Believe me I've spent many many evening with a caliper pencil and paper going over all the materials and dimensions. There are some things that became apparent with a mock-up that just isn't apparent when drawing them on paper. I did some work on cad but I'm not going to spend months with drawings when I have already made one and now can see how it's done.
My suggestion is to make a model, mock-up, what ever you want to call it and see how it works. Originally I was going to make a single side holder but I slowly realized that what came before me was what I was slowly modifying my holders to. I figured that I was trying too hard to reinvent the basic film holder. I'm making double side holders.
I hope you can read this as I've just returned from the North Cascades National Park. I've been house bound and it was nice to get out. I saw some things I want to photograph with a large format camera. I have 100 sheet box of Kodak green 11x14 film here and that's what I'll probably use for the most part. I will use some regular film too later on.