Dave Rees at The Gadgeteer, after realizing that his Force FX lightsabers really weren't meant for sparring, decided to create his own so that he and his son could battle. He's now sharing how to do so with you, the Star Wars and lightsaber loving audience. Click here!
Oh, and please do be careful Dave! Father/Son duels don't tend to end well in Star Wars!
UPDATE: Along the same lines as the post above...ClubJade.net via io9.com points us to the below video showing how you can make your own lightsaber quite affordably.