Not much to them, I turned down some scrap acrylic stock to the same diameter as the 901 outer casing. Turned down the shoulder, until it fit snugly into the end of the battery tube. Because the led is almost flush to the end of the battery tube, you have to hollow the center. I used a 1/4" spotting drill to cut this recess. Cut it off, and lapped it on a piece of 320 sandpaper to clean up the face. You also have to cut a small hole in the side wall to allow air to pass through if it is an automatic battery.
Ok that sounds worse than it is. Its really only about 2 minutes worth of work.