I appreciate and enjoy all those but they can be a little time-consuming/expensive/requiring skills you may not have.
So I made this one for under $15 and in under 15 minutes. It's perfect if you need to throw together a costume but don't have a lot of time and/or money (or, in my case, craftsmanship skill).
Here's all you need:
1. A battery-powered, short string of blue LEDs (example: http://www.littlebrightlights.com/site/1435548/product/AC-100B). My string was Philips brand, 3 feet long (if I remember correctly) with 18 lights but really, any brand and any amount of lights under 20 would probably work just as well -- I got mine at Target for $8. Each of my individual lights are just about 1/2-inch long and 1/8th of an inch across. Try and find lights that are on a string of a shorter length, with as little space between bulbs as possible (this information should be on the box). The lights are the most important thing, not the length of the cord.
2. A hockey puck-shaped piece of white Styrofoam (1-inch thick, just under 3 inches in diameter). I got my piece by cutting the bottom off a conical piece of craft Styrofoam I already had. Packing Styrofoam will probably be too brittle to cut and it also has less opacity/reflectivity. I'd recommend using the lighter, craft Styrofoam that has the crystallized look and is made to be cut (Michaels is a good option).
3. A pen or marker.
4. A toothpick.
5. Tape (glue would work, too). I used 2-inch wide clear packing tape but any kind would probably work.