Simple tshirt cannons

Our team just had its FIRST meeting of the year, and we are happy to have a ton of new people. With that in mind, we would like to start off the year by making a tshirt cannon. We don't need anything as complex as 254's Shockwave (My favorite robot of all time. ALL TIME.), but we would like a simple 2 or 3 barreled pneumatic cannon on a drivable chassis.

So my questions are:

1. What kind of materials should we get for the cannon? (PVC or no?)
2. About how much time would building it take? (just building, not bugging/programming)
3. About how much money would the cannon itself take? We have the rest of the parts.

Thanks!