Making an Airzooka-style air cannon is fairly simple. It requires a cylinder with a hole in one end and a diaphragm in the other end. This solution using a cheap plastic mug cost me less than $5 to make. While it doesn't have the force and distance of the brand name Airzookas, it also costs a lot less.
• A single-walled cheap plastic drinking mug (with handle). The larger the better. You want one with a screw-on lid as it makes it easy to secure the diaphragm.
• Rubber bands (assorted)
• A good quality plastic bag (like the sturdy gift bags from a department store.) Cheap plastic grocery bags won't work.
• A little bit of duct tape.
• a utility knife
• a hole cutter
• a dremmel tool (with general purpose cutting bit and a drill bit)
Step 1: The Front
Cut a big hole in the exact center of the bottom of the mug equal to about 2/3 the diameter of the bottom. I used a cutter made for installing deadbolt locks in wooden doors simply because it happened to be about the right diameter in relation to the mug. In hindsight, I probably should have made the hole just a slight bit larger.
Just outside this large hole, drill two small holes on each side and cut slots through to them (from the large hole) with your knife. This "tab" is where the rubber bands get attached. (See diagram.) It should be a tight fit for the rubber bands. You don't want them slipping out/off. You also want these holes far enough away from the main hole so that the tension of the bands doesn't bend that “tab” allowing the rubber bands to escape.
Step 2: The Back
Using your hole cutter or dremmel, cut away all the inner plastic from the lid, leaving only a plastic ring with its threads. It's important that you not damage the threads or compromise the strength of the ring. This will be used to lock down the plastic "diaphragm."
Next, cut a section of plastic from your bag. I used the corner of the bag because it tends to taper to a point, which is what you want (a cone). You want the plastic to come as far out of the back of the cannon as possible and travel as far into the cannon as possible. This will cause it to push more air. However, you don't want it to go so far into the cannon that you can't get it out again. If you don't want to use the corner of the bag, cut out a large circle, at least twice the diameter of the mug. (If in doubt, go larger. It's better to start large and trim off the excess when you're totally done.)
Now, reinforce that corner of the bag (or the exact center of your circle of plastic, if you've cut a circle) by sticking a small piece of duct tape on both sides. (This helps reinforce where the plastic connects to the rubber bands so that the pressure of the rubber bands doesn't tear the plastic.)
Center the plastic over the opening of the mug. Push the plastic into the mug almost half way and carefully screw down the "locking ring" you created from the mug's lid. You want to have enough of the plastic bag loose to flex in and out of the mug. Take a look at the backlit photo in this step to see how the air cannon looks at rest to get an idea of how far into the mug the plastic should be. (In the next step there's a photo showing how far out the plastic will come.)
Step 3: Attach Rubber Bands
Attach skinny rubber bands to the front of your air cannon around the "tabs" you created in step one. See the picture for reference.
Next, cut two slits side by side in the duct tape square in your plastic bag diaphragm to allow the rubber bands from the front to pass through the bag and get tied to the “handle” in the back. In my case, since I was cheap and in a hurry, the "handle" is just another, thicker rubber band. The idea is to pinch the bag between the rubber bands and the handle by passing the front rubber bands through those two slits and tying your "handle" band around both the front rubber bands and the center of the reinforced plastic diaphragm.
You also want the front rubber bands to have a little bit of tension at rest. You'll have to experiment to find the right length rubber bands and might also have to adjust your plastic diaphragm.
Step 4: Fit, Finish, and Fun!
Adjust the plastic diaphragm as needed to achieve satisfactory results and then trim remaining plastic.
Now go blow air at stuff! I'm pleasantly surprised at how this turned how for the price. At 10 feet, very good results. 15 feet seems to be about the maximum reliable distance. It will push air beyond that, but at slower speeds. Accuracy also becomes difficult beyond 12 or 15 feet. I have seen results up to 20 feet with a 3 second delay. Compared to the brand-name Airzookas, less air is being moved of course. But targeting (since this model is smaller) is actually easier. Enjoy!