This is a stubby flashlight that holds twenty-seven 5mm LEDs and has a cutout on the bottom for a 12mm size pushbutton switch. Both the LED and switch mounts are removable and are secured via a number of small screws.
You'll need to figure out how much voltage you'll be using and the appropriate resistor(s) for your LEDs.
UPDATE: I've included a three-switch bottom plate for those using RGB LEDs (or similar). I've also included an extension that can be printed in case you need to make your flashlight longer (just glue it to the main body).
Twenty-seven 5mm LEDs are required along with a 12mm sized switch and several small screws. Also a battery holder and appropriate battery and resistors are required.
The LED holes are sized at 6mm to give some wiggle room when installing them. Simply tack them in place with glue or similar.