This LED disk holds 20 LEDs and has enough room for the required resistors (10ohm) if you use a 9V battery. Need some hot glue to insulate the connections but it works great.
It does have a retaining ring in order to install flush or to better mount and 2 hols on the back side to run a string should you need to tie it to something.
the Disk diameter is 40mm and the retaining ring diameter is 48mm. It sicks out behind the retaining ring 11mm and in front of it about 5mm.
The LEDs are very close to the front of the disk so you can use most any color plastic and they will show through.