foam adjustable snoot. It can also be used as a bounce card or if you turn the black side in it could be a gobo. And for a DIY project it don't look half bad. Better than a cereal box snoot when working a paid gig.
This not so great photo was taken with the Strobist cereal box snoot (sans straws) and is included to show how you can really narrow a strobe source down with a snoot.