3D stereo achieved using polarizing filters over the projector lenses, and matching polarized glasses
Some sources for polarizing glasses are:
Linear polarizing filters can be bought from Reel 3-D, or photographics filters can be used (e.g. 72mm Hoya PL).
Circular filters can be bought from American Polarizer, or Edmund Optics.
Because of variations in projector design, a custom mount must usually be built. See the GeoWall site for info on some mounts that others have made.
Using mirrors can affect polarization - keep angle of reflection small.