I like the idea of using solid graphite. And the setup you have is definitely in the right direction. It makes sense to have the second aperture a distance away. Since there is a good dark band between the beam and the first diffracted band, your second aperture doesn't need to be as precisely placed if it's just blocking the band.

I can't remember the formula for the angle of diffraction through a hole, but I do remember that it's dependent on the wavelength, so you've got one of the toughest possible scenarios with this.

Unfortunately, this little revelation adds parts that need machining and I can't for the life of me figure out how you can get it so the end user can calibrate a multi-step attenuator easily, since you have to aim the laser exactly through both of the holes.