Charles Town, WV
Its a tight fit but it actually goes easy if you use a socket wrench with a socket that has an opening that is smaller than the gumball but large enough to cup the gumball. Place the gumball right up to the spot to insert and use the socket wrench to pop them in. This worked for me and did not break or chip the plastic. There are other threads on this if you run a quick search which I am sure is where I found this trick.
https://pinside.com/pinball/forum/topic/getting-gumballs-in-a-twilight-zone
