I guess there are many ways to get free-play on non free-play games. From setting the threshold low so every game will win a free game to shorting the coin switch with a key. On one of my games someone added a flipper switch to the front of the cabinet that is connected to the coin switch. This works well but I don’t like defacing the cabinet front. See first two photos.
So on my other game I added a doorbell switch to the inside of the cabinet. Fortunately the wiring harness was made for three coin switches but only two are used. I simply extended the wires to the new switch. See photos 3 and 4.
This does require opening the coin door to get free games but it can be removed easily and is no damage to the cabinet. Also I set the center coin slot for 5 games per coin so I only have to press the doorbell button 8 times for 40 games.