Please excuse the number of steps. It is really easy and can be made within an hour and from bits lying around the office, hence very cheaply.
Step 1: The tools and materials list
1 pair of pliers (needle nose would be best, where available)
1 pair of scissors (yay for scissors)
Corrugated card (approx 6"X6")
Ballast (I used a bunch of batteries from our recycling box)