As promised, I have created an adjustable slit for the spectrophotometer. This simple design still uses two single-edged razor blades, but makes one moveable so the width of the slit can be adjusted. Pegs on the blade holders are melted to hold the blades in place. A 3mm screw is used as the adjuster and a ball-point pen spring is used for tension. The left-hand blade holder and the screw mount are glued to the housing; the right-hand blade holder floats so the slit can be adjusted. I had to cut a groove in the floor of the housing to provide clearance for the new assembly. If you take the height of the assembly into account when you make your housing, this won't be necessary. (I'm just too lazy to make a new housing right now - maybe later?)
The stl files are posted so you can make your own (just like the rest of this project!). The design files in FreeCAD format are also posted, in case you want to modify anything.
Next step is to modify the control program and the data collection program to make use of the narrower slit. I'm thinking that using writeMicroseconds for fine control will work.
As an aside, it occurs to me that the adjustable slit could be controlled with a stepper motor to provide variable resolution. A narrower slit and finer movement of the light source could be used in a region of interest, while a wider slit could be used elsewhere for faster data collection. Hmmmm.