I've just now put together these neat little devices - Printed Circuit Boards with 40 LEDs; side fire on the top perimiter to send light into the pop bumper cap; underside to light the play field; and a ring of center LEDs that can be set to "flash react" with ball strikes based on vibration or to just stay on. Brightness is also adjustable. These come in a variety of colors and just plug-in with no wires, no glue. I love modding with LEDs and was never happy with single LEDs for pop bumpers. These white circuit boards sort of "glow" from the LEDs and can be used with existing caps or clear caps.
I've put more information on the following link:
Note - these circuit boards do add about 5 mm hight to the pop bumper. Not recommended for very tight spaces or where pop bumper caps are "cut into partials" on some games.