I used standard blackout cloth. The final result seemed very close to a 1.0 gain matte white. Since I didn't have a matte white to compare it to, the link I have that goes to the BMXTRIX website actually compares the image to a $4,000 High Contrast Grey Draper Screen (.8 gain). The DIY screen is clearly brighter and perhaps a bit more washed out in the shadows. The projector used for that particular demo was an Panny AE700.
Standard blackout cloth though from JoAnn Fabrics. Used the 'white' side.
EDIT: Obviously the photo above is a Photoshop job done with the lights off in the room, then with the lights on. The projected image itself is 100% untouched, but was laid on top of the 'lit' room. A more accurate image would be with a completely dark room (black) with the exact same image.