You don't want a standard 'COA' (they are worthless), and you don't want a 'generic letter' either. You want something like GKvfx mentioned- letters specifically stating what these pieces are, and where/how they were used. Not 'I certify this was used in Star Wars ROTJ', but 'This piece of wood was a section of Jabba's Sail Barge left behind after filming, recovered by me and my friends from the desert and verified by [prop master/etc].' (Example, you get the idea...
) If possible, include as much detail as possible as to how it was authenticated if it's not really obvious. When you print out the LOA (Letter Of Authenticity, what you want in this case), include, on the same paper, a photo of the piece in question, and a means to contact you that will work for the foreseeable future should the new prop owner ever need to ask a question about it. Have the prop master sign that... (And, if you are getting it signed in person, take a picture of him with the prop and the LOA if he's cool with that...)
That makes for a great provenance and history of the verification process. That's what is important. It reduces any doubt about the pieces, and makes a cool story to go with them...
As for COAs, if you are determined, they all look different anyway... Nice paper stock, fancy border, some kind of 'security' feature (hologram sticker, gold embossed sticker or something)... But if you aren't willing to _personally_ stand behind the COA for the rest of your life regardless of who owns the prop in the future, it's just a pretty piece of paper... The documented history is far more important.