We did it for a couple of years until they figured it out and started to conver the arcade cabinets with transparent plastic.
At the same time they also drilled holes at the back of the machine for ventilation as the rest of the case now was sealed in plastic.
We found out that using a bamboo stick you could press the lever that register when a coin has been paid into the slot.
That made them relocate the holes for the ventilation to the top of the case instead of the back so we couldn't get the lever anymore. Or so they thought. haha
We discovered that by pressing a coin up the return slot — the one where you get your coin back if it isn’t accepted — you could also trigger the lever for coin registration and the free gaming continued.
Eventually they put in sharp screws into that coin return box so you would cut your finges.
After that we got a SEGA. Was great fun :)
In fact, that could be why some of the machines weren't better protected against that stuff in the first place, right?
And the technicians "improving" the machines -- maybe they had a good time too, I'm wondering. @TowerTall and friends made their job more interesting / fun?