I made my first one with a film canister, worked pretty well. Drill a small hole in the centre of the cap, 1/16" is good, cut a neat round hole of about 3/4" diam. in the bottom, don't cut the bottom off as suggested elsewhere, that way you preserve the stiffness off the canister. Get some thin copper wire to make cross hairs and glue it over the bottom hole so they cross as near the exact centre as possible. Voila!! instant collimator
Cheers
Bill