You will need:
- A thick plastic canister (the thicker the better)
- A tube.
- A piezo.
- 2x screws.
- Some plastic coated wire.
- Sandpaper (optional)
Important: The canister will have to deal with a strong pressure so you must avoid using plastics wich can easily shatter, a hinge lid will open releasing the pressure if the barrel gets stuck with no consequences for the person who uses it, a threaded cap won't let the pressure go out, even most of the canisters can easily hold the pressure it's recommended to use one with a hinge lid.
The bigger the canister it is the smaller the explosion will be, this is because the alcohol requires a small volume to quickly rise up its pressure and temperature (Ideal gas law), the optimal volume is around 15cc and 30cc, to calculate its volume put the canister in a scale and zero it, the weigh of the water in grams will equal the volume in cubic centimeters.
How to chose the right barrel: I can't recommend an specific barrel because It depends on the size of the canister you are using, when choosing the tube remember the alcohol power has limitations, If you use a wide tube it has to be short to avoid the suction of the gases cooling, if it's thin it has to be long to use all the energy, with this said I would recommend a barrel which has an inner volume between 1/2 and 2/3 of the canister volume.
A long and thin barrel will shot small projectiles at a bigger speed, a short and wide barrel will shot bigger projectiles but more slower.