The Simplest Light: Battery and Bulb
Use a 1.5 V flashlight bulb and an AAA battery.
Solder wires to the battery and bulb.
Twist wires together to turn it on.
Simple an easy and fits in a BT-20 payload section.
Source: Inexpensive lights and sockets: Miniature Christmas lights. A string of 100 lights in series yields 100 - 1.2 volt lights and sockets. (Note: bulbs short on failure)