It's so easy to make that, apparently, I could do it!
Apologies for poor quality pic-churs...
Step 1: Gather Supplies
One: A cereal box (If you are using a powerful gun you made need something stronger e.g a wooden wine box...
Two: several plates and bowls of different sizes
Three: A big black marker
Four: A scissors
Five: A sheet of A4 (printer size) paper
Six: Sticky tape, duct tape, scotch tape, ANY TAPE!!
Seven: An Airsoft gun