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)
Step 3: Now shape them
At the other end, make a dog leg. Try to make it on the same plane/axis as the eyelet.
EDIT: Ignore the dogleg. Instead bend approximately 1/2" of the non-eyeleted end at right angles (90 degrees) to the eyelet. All will become clear very soon.
Step 4: The axle
Step 5: The arm
Make a small hook at the other end.
Step 6: The trigger
EDIT: See the adjustment I made to the firing mechanism in the final step. It works so much better.