I had found some electrical plates that worked really well and look good.
Get the shiniest ones you can find at some Home Improvement place that
has a large variety of plates such as Lowe's or Home Depot.
Use the two screws that usually come with them to go through a flat bar of
about a length about an inch longer than the bill acceptor opening.
Drill a couple of holes into the flat bar for the screws.
Use a couple of nuts to hold it all in place.
If you fear it slides around too much, use a little square piece of double side
sticky tape like they use for plastic window covering.
Stick the square sticky's on the back of the metal bar and it
will adhere to the backside of the opening.
The bar will not slide around and it will hold your electrical plate in place!
Another cheap solution is a black piece of lexan or plastic that can be adhered with GooP
to cover the hole. Put a nice cool sticker or something if you want to "dress" up the black.
Maybe a nice game logo perhaps? What game do you have? Or a nice photoshopped IGT logo?
Goop comes off rather easily should you want to add a bill acceptor later on down the road.
It dries like rubber caulking and peels off the chrome door if you want pretty easily.