If you are concerned about exact copies of your layout/design I would remove any images of the unpopulated playfield from the web. I remember seeing a picture of the printed overlay posted on photobucket or some site like that. Don't make it easy for them to copy your hard work.
Making a short run of pins or a conversion kit to sell on ebay or elsewhere could be risky, but you could be commissioned to make one off machines for people who ask, just like any other artist. So in other words you make them to order like a pizza mmmm pizza.