the company I work for did a similar giveaway at the SEMA show three years ago.
My suggestion would be to have keys made (40) but only one of the keys cranks it. 39 almost correct keys 1 that works.
Make sure you make extra dummy keys and triple check that they do not work. You do not want to have two that work.
39 confirmed non-working keys and the one working key are place the in a bag and have the 40 people pull blind.
This is a good idea. But make sure no one sees the working key.
I was a participant in one like this, giving away a car. They made a big production over showing the working key going into a jar. They held it up for everyone to see. They were also multi colored keys with swirl patterns on them. One guy ran out of the crowd up to the stage to get a close look at the key. Wouldn't you know it, he was about the 10th person called to draw and he picked a key, looked at it and threw it back and picked another. Same thing with that one. Finally, he looked in the jar and picked a key out, tried it. Guess what, he drove home in a new Mustang that day. He said he looked at the swirl pattern and the pattern of the cut and new the first two he picked were wrong and since they didn't stop him, he kept looking for the right one. Everyone cried fowl, but what are ya gonna do?
Just make sure it is a blind draw and only one chance to draw.