This was a design I worked on back in December of 2016, I took the inspiration from a ThorLabs design, which gave me the idea to make one using a mechanical micrometer a means to manually adjust the entrance slit in increments of my choosing.
Namely, in the range of 0.005/mm, it works on FreeCad v0.16 using the animation simulation, but I am still trying to work out some tolerance issues that are plaguing this design, mainly because of the 3d printing material and techniques.
I'm going to upload the mesh/.stl files anyway in case someone can tweak it and make it better, don't worry there is no license or anything, as all my work here is provided freely, just relying on an honor system of giving credit where credit is due.
The blue hexagon, are 2 Gillette utility knife razor blades, which I examined the edges with my microscope (100x) and the edges are surprisingly even, with fewer jagged edges than normal blades, so both edges come together with a very tight fit.
I have the mechanical drawings available if anyone is interested, just let me know and I will email them to you.