Yelobird, the approach you described for changing the phone is basically what I did the first time I installed my mod without the cake pan. This was before I knew the cakepan existed and only found the phone from the Bat-cave playset. As Yelobird mentioned, the bat-cave playset phone is the one to get. It has the right amount of translucency to really light up well and its the perfect size for the cake pan. Although its expensive to buy that set just for the phone you also get a bunch of cool toys like Shakespeare bust, shark repellent, bat-zooka, bat-computer, bat-a-rangs,and even a 2 headed Batman/Bruce Wayne action figure!
Maybe someone with a 3D printer (looking at you Yelobird) could make a translucent bat-phone just like the one in the playset for a lot cheaper?
And I also thought about the fact that the bat-phone at Wayne manor does not have a cake pan on top of it. But my rationale is that it just looks cooler with it! If you really wanted to justify it logically you could assume Alfred saw the cake pan cover on the bat-phone in Commissioner Gordon's office and decided that having that dome keeps the bat-phone dust free and cleaner so he ended up getting one for the bat-phone in Bruce's study.
The part of the original approach I didn't like was that the Stern LED board was now mounted under the plastic instead of on the big chew toy phone above the plastic. When I attached the Stern LED board under the plastic using a nut/bolt I was worried that the ball traveling at the just the right rate of speed and maybe a little bounce might possibly nick the head of bolt or the edge of the LED board. Not sure if that would actually happen or not but pinballs with nicks in them end up destroying playfields. So once i saw the cake pan option I figured out an alternate way to light the phone up and keep everything far away from the ball.
I think I found a better way though once you add the cake pan. The phone that comes with the cake pan is glued to the top of it. Just rip it off and toss it in the garbage. It not translucent enough and it no more detailed than the larger Stern version.
The bottom of the cake pan can be popped out fairly easily using the tip of an exacto blade.
Drill a 3/16" hole right through the middle of the top. This is just big enough to tightly wedge a 5mm red LED in there. You also drill an access hole on the side of the cake pan so your wires can come out the side instead of the bottom. This way you have a nice flat bottom to attach to your plastic.
drill led hole (resized).jpg
drill side hole (resized).jpg
I reused the harness from the Stern LED board. You just need to wire a resistor in with it the regulate the current. Calculate the required resistor using something like this:
Most generic red LED's will have about a 2v voltage drop and run on 20ma. So if you used this LED:
You would need a 150 or higher resistor. As the resitor value goes up the LED would get a little dimmer as the current would decrease. I think I used a 200 ohm resitor when I did mine.
I disconnected the Stern LED board harness from under the turntable so it would be easier to work with on the workbench. You need to un-solder the +5v and ground from the Stern LED board and solder them to your LED/resistor. Before you solder to your LED, feed the +5v and ground wires through the hole in the side of the cake pan. Solder the +5V from the Stern wire harness to the longer leg of the LED (this is + side or anode) then solder the ground wire from the Stern harness to one end of your resistor and the other end of your resistor gets soldered to shorter length leg of the LED (minus side or cathode). you should add some electrical tape or shrink tubing on the LED leads so they don't short out to each other.
Using a little dab of super glue you can reattach the bottom of the cake pan. For the phone I use "glue dots" to glue the phone to the cakepan over the LED.
glue dots (resized).jpg
Then I use some more glue dots to attach the dome of the cake pan on top. I also use the glue dots to attached the base of the phone and the Shakespeare bust to the plastic on the turntable. The glue dots are very strong and nothing has fallen off yet. They are also removable if need be.
phone harness (resized).jpg
I decided I like the larger Shakespeare bust vs the smaller one that comes with the batcave playset. You can get the larger bust at the same place you get the cake pan for $6.
If some of you need help with this and want me to make up kits so you just plug and play install it I could provide that.