I haven't gotten as far as making accessories for him, but I have been exploring other outfit options. Build a bear outfits fit him great! I have several outfits from there to use on our upcoming trip. So most of their accessoires will work - though the hats seem to be a challenge.
I also have picked up a few preemie baby onsies that fit very nicely also - and pretty unique. I also take the plain tshirts (from build a bear) and print iron ons off my computer to make little shirts that fit different occasions or trips. He has a Disney name badge shirt, and one with a monorail that wraps the shirt.
I wanted him to stand out a little while we are in the parks to make him easier for my daughter to recognize. She has special needs and has a hard time communicating and uses Duffy as a way to interact with people.
Oh - I did purchase a sound box from Build a Bear and put it in Duffy's arm for her also.
Sorry I rambled so much!