Having built over a dozen LF and ULF cameras, I can give a couple of points that you need to consider.

1. the ONLY exact measurement in a LF camera is making sure the film plane is at the same plane as the ground glass. Every other measurement, size, movement, etc. is up to the builder.

2. The best way to build a "first" camera is to start with a commercial film back. Buy a 4x5 or whatever size you want film back with the glass and film holder already done, and build your camera around this.

I have built cameras out of everything from foam core to plate aluminum, and having a starting point is always a plus on any build. When I don't use a pre-made film back on a LF, I tend to make 3 film holders specific and custom to the camera. I have never made a film back that I felt was precise enough to use commercial film holders on, and it's again easier to just make them specific to the camera.