Blunty made a Plastimake enclosure for an LED "Joule Thief". It diffuses the light nicely and makes the circuit a lot more durable. A "Joule Thief" is a simple circuit that can run on a flat AA battery for hours.
Plastimake and LEDs go really well together. Plastimake is slightly translucent which creates a neat glowing effect when an LED is embedded inside. Although some LEDs can get warm, they won't get hot enough to soften Plastimake unless they are extremely high powered modules.