My son is a fan of The Nightmare Before Christmas movie and wanted to dress up as Barrel for Halloween, and he wanted me to dress as Jack Skellington. The only problem was no one makes a Barrel costume. So I had to step up and get creative. I made a mask for him and finished off his costume with a cheap skeleton body suit, gloves, and a little make up and temporary hair dye.
The following is a summary of how I made it, so others can do the same. The entire process took about 6 hours from start to finish.

Supplies

Supplies I used for the mask

About 7oz of thermoplastic pellets - 6oz for the mask, and 1oz for the teeth
Yellow water color paint to dye the plastic yellow for the teeth. It would probably be easier to use pre-colored pellets, but I chose to color my own.
A black sharpie to outline the teeth and eye holes.
Clear hair hoop to hold it on the top of head (optional)

Tools and utensils used

A small sauce pan (or anything to heat water in).
A plate (Any flat surface non stick surface will work. A dinner plate just happens to be the perfect size circle.) to shape the plastic into a round shape.
Scissors to cut the plastic.
A shot glass to cut eyeholes.
A small ball to shape the mask.
Wax paper to keep the plastic from sticking to the ball.

Step 1: Shaping the Mask Into a Circle, and Cutting Eye Holes.

Place about 6oz of prewarmed thermoplastic pellets on a plate or other flat surface and work it into a flat circle ( like a small pizza crust).

While still warm, press a shot glass into the mask as a reference for where you want the eyeholes to be. You need to press hard enough to make a good impression to make it easier to cut out the eye holes with scissors after the plastic cools.

Let the mask cool until the plastic is solid again. You can use cold water or ice to speed up the process.

Once cooled you can cut the eye holes out with scissors. It doesn't need to be perfectly done because any imperfections will be fixed in the next step.

Using scissors you can trim the edge of the mask into a circle.

Step 2: Shaping the Masks Final Shape

With mask on a dinner plate, pour enough warm water on it to make it pliable (clear) again. At this point you can fix any problems with the eyeholes.

Put wax paper over a ball (child's soccer or basketball, in my case I used a small medicine (exercise) ball. For adult size, use a full size soccer ball or basketball). The wax paper will keep the plastic from sticking to the ball, and yes it will stick to some plastics..... I know from experience.

With the mask warm and pliable, place it over the wax paper covered ball to give it a semi-globe shape.

Let cool.

Step 3: Shaping and Adding the Teeth

If you are coloring your own plastic, warm it to a pliable state then flatten and place a drop of coloring in the center. Fold the plastic over on itself and kneed it with your fingers (you might want to put on some glove to keep your hands from getting stained). Repeat this process until it is the color desired.

Once colored properly, place it on a flat surface and flatten into a long strip about 1 inch wide and 1/4 inch thick (this doesn't have to be exact).

Wait for it to cool. Again, ice or cold water can be used to speed up.

Cut out teeth shapes from the plastics. It is best to look at a still from the movie to get the shapes and size right.

With the mask cooled and teeth cooled you can attach them to the mask by heating up the underside of the teeth and pressing them into the mask. You can also use superglue as an alternative.

Step 4: Final Touches

Using a black sharpie outline the eyes, teeth and draw in the nose holes.

At this point the mask itself is finished. If you want to add a clear hair hoop to wear it on top of the head, use thermoplastic to attach it. As an alternative you can use string, elastic or ribbon.