.....I take it no exchanges for you on prizes then, eh? .....
That would open a door you don't want to go through. When you give a prize, it is THAT item they won. Unless the prize is for "A custom knife of your choice/design, then the one they won is the one they get.
As to the "Knife of your design" option, I did that about five years ago. I would donate a custom Sgian Dubh to our Scottish Society's raffle every year. The winner would usually ask if I could modify it by adding some feature to fit his avocation ( like a piper emblem) or his clan ( like a clan crest pommel). I would tell them that I would have to make a new, custom knife to do that. So, I decided to offer a custom Sgian Dubh of value around $400 as the prize. We sell twenty $20 keys for the "Lock Box" raffle ( only one key opens the box and gets the prize inside). The winner was a nice guy, but I had fits trying to pin him down on what he wanted. After six months, I finally made him an ivory handled Sgian Dubh with his clan name engraved down the side. He was quite happy, but I never did that offer again.