A long post from Instructables (Simple Variable Neutral Density Filter) invites us to build easily a Neutral Density (ND) filter whose density can be adjusted (using only two polarizing filters).

The simple and neat idea is to flip back one of the filters in front of the other. Correctly aligned, they become nearly totally black (a very very dense neutral grey color); In opposition, they give a light grey filter. It can be set continuously between these extremes with a slight rotation.

The key point is to unmount tidily one of the filters to flip it, then to screw one onto the other.