Over on candlepowerforums, some of the things people used to do was to use the Palm Pilot screen protectors, or tape Scotch tape (I believe they had a favorite for the effect - that would call for a search on CPF) over lenses. You could perhaps get a Minimag accessory kit, with the colored and clear extra lenses and the rubber bezel cover. Then, you could put diffuser material of your choice on the spare clear lens, and keep it on with the bezel cover, then remove it when you want an undiffused beam.

Good luck

Dave
(my kids demanded I add a smiley for each of them)