Quoted from yonkiman:

Hi Folks. Here's a photo of my contraption:

Should be pretty self-explanatory - the big red circle (about 2x the diameter of the flipper button it's pushing) is the plunger that pushes the button.
This doohickey meets all my design goals:
- Fast and Easy to Set-Up: You just plop it down on the glass. The weight of the 8 D cells and the no-slip rubber under the battery case keep it in place. The large plunger head allows for varying button placement for different pins from different manufacturers (has not been confirmed in the wild yet). Couldn't be simpler.
- No lag: Using a solenoid means the second you press the actuator, the flipper is pressed. (OK, there are a few ms of delay but it's as instant as I believe is possible.) The only drawback is you won't be able to use the staged feature of multi-stage flippers, meaning in a 2 left flipper scenario, you won't be able to have the bottom flipper energized and the upper flipper off - either both will be energized or both will be off. I'm designing this for ease of set-up for 95%+ of the pins out there.
- Battery life is long. 8 D cells hold a lot of energy. I have implemented a "Hold" mode in the electronics that sends less power to the solenoid after flipping. If you play two hours a day every day, new Duracells should last a year.
- Rechargeable option: Just use rechargeable D cells. Circuit will work fine even with 1.2V rechargeables. Plus in an emergency you can always buy standard D cells.
Now the only thing I *haven't* completely figured out yet is the trigger/switch for the player's fingers. My goal is something that attaches to the player's middle finger, and fires when tapped against the side of the cabinet. (Obviously it will be covered in soft felt or something similar so it won't damage/wear out the side of the cab.) I have an idea for something using a micro switch but my 3D printer broke right after I printed this last prototype and have spent most of my free time since working on the printer. Think a solution with a leaf switch might be good, too.
ANYONE'S/EVERYONE'S IDEAS FOR THE FINGER TRIGGER ARE WELCOME! My goal is ease of use. In a multiplayer scenario, when it's the player's turn, (s)he just drops this on the glass and starts playing. Maybe 3 seconds of setup. I'm trying to avoid suction cups, any sort of non-instant alignment, etc.
I (and my 3D printer) will be out of comission for most of this week, but I hope to resume work next weekend. Would appreciate any ideas/feedback I get before then!