What if a Grasshopper Drops in! or some kind of other element in the external environment, bingo ball is in! , i will suggest double checking the result, Im thinking how it may happen.
... well I hope there arent too many insects in your arcade or no kids will come to play.
Double checkign wouldnt be too hard, just have 1 beam straight on acrossed the score hole and another offset so that it takes teh large ball going through and setting off both to cause it to score. I doubt a grasshopper is big enough to cross both streams, maybe a moth or butterfly could, but I doubt theres too many of those in the building. If someone gets a free score, well aren't they lucky.