I just bought a Plinko. To fix the gap, I put a little something between the offending half of the board -- in my case the bottom -- and the brace at the back. At the suggestion of a friend, I chose an index card cut small enough to fit, and folded it until it was just the right thickness. Presto! The two halves are more seamless, and there's been no more flyouts since.
Also, unauthorized use: if you have any half-dollar coins around, they work very well as chips. The included chips are a good deal heavier than that, and travel pretty fast. Half dollars are about the same diameter, but without the added weight, they achieve more of that fun pause-and-bounce the chips on TV do.
-Jason