First step in making an excavation for the film holder is laying out the recess. To do this, I center the film holder’s window over that of the insert and strike a line:
When the area is marked, I turn to the router and a rabbeting bit. I size the bearing to be just a little larger than the marked line so the holder isn’t too tight in the back. Then I rout the recess. The recess needs to be a little longer on the end, and open on the front. I rout these areas free-hand using a hinge mortising bit. The edges and round corners are finished using a chisel:
A quick check shows we’re on the right track:
The film holders have a double light trap on the darkslide end:
To Make the excavations for the light trap, I use a Dremel with a router base and round burr cutter, and rout the open end of the insert free-hand with the router and hinge mortising bit. The edges are again clean up with a chisel.
A final check shows the light trap is a perfect fit in the new insert:
The next step is to build the ground glass holder. This is a delicate project, as the pieces are on the order of 5/16″ thick, and the rebate for the ground glass has to be placed at the film plane. I’ll start by planing my cherry this afternoon…
Scott Perry Photography 
I’m glad you keep these pages up. I am on perhaps my sixth attempt to make this contraption: I keep getting just a bit further before I make some Fatal Error (cut too much off, get happy with the router, any number of things that point to inability to use a ruler). I should have just asked you make me one. Even with my time being worth nothing and scrap materials, it’s still frustrating and wasteful. I think my Big Lesson for this go-round is less power tools, more hand tools, specifically the table saw but perhaps the router as well.
I have had some equipment issues as well: your advice to use the table saw to mill those rails and carve out the mortices doesn’t work very all as my blade seems to leave an angled edge. The rails were not of an even thickness and didn’t fit well and the mortices were more like a V groove than a square mortise. Those were done on the router this time and are much better. The frame is gluing up and perhaps tomorrow I’ll see if I can cut out the slot for the film holder without making a mess of it.