When the trick-or-treaters show up on your doorstep tomorrow, notice how few are wearing flimsy plastic outfits with heat-sealed seams. Plastic costumes onces were ubiquitous, and their disappearance is a fact for which Bob Pickens takes some credit. Sixteen years ago, Pickens and two associates started a company they call Disguise. Today the Poway enterprise claims to be the world's largest manufacturer of Halloween apparel. Two other large manufacturers also have offices here, making San Diego, according to one observer, "the Halloween capital of the United States."