In order to build a shotgun microphone, you'll need to gather a long list of supplies. The bulk of the microphone will consist of a total of 56 feet of 3/8 inch aluminum tubing. You'll also need a crystal microphone cartridge, aluminum stripping, camera tripod, funnel, microphone cable, screws, and glue. Some basic hand tools, such as a screwdriver, hacksaw and measuring tape will also come in handy. Be aware that your microphone will end up being quite large, due to the number of aluminum tubes required. If you plan to use your shotgun for spying purposes, you're in for a shock when you see its size.
The aluminum tubes you purchased will be used to create the main body of the microphone, and serve to detect sounds far in the distance. Each aluminum tube you cut will resonate at a certain frequency. Cut your aluminum tubing into individual tubes, ranging in length from 1 inch to 36 inches. You should end up with 36 tubes total. Glue the aluminum tubes together so that they look like a symmetrical bundle, with the longest tube on the inside of the bundle. Each subsequently shorter tube will then be arranged circularly around the longest tube. The tubes should be parallel to one another in order to achieve the proper directionality of the microphone.
You'll have to assemble and attach the microphone cartridge unit. Run the microphone cable through the narrow end of the funnel and attach it to the mic cartridge. Dab some glue around the inside edge of the funnel's mouth, and fit it around the flat end of your aluminum tube bundle. Screw some aluminum strips to the bottom of the tube bundle, and attach the strips to your tripod. If your metal working skills are lacking, you might try using the tripod attachment of an old camcorder instead of aluminum strips. Attach the bundle the tripod.