I used a lathe to make an adapter so I could get this JUST the way I wanted. But you don't need to. The basic components are a barrel, a modified sprinkler valve, and an air tank with a tire valve to fill it. Shape doesn't matter, and size isn't really critical, it will be tons of fun as long as the air tank is close to as large in volume as the barrel, once you get to double the volume of the barrel any larger doesn't do much.
If you gather the parts as you find them on sale or at a garage sale you can build this for <$40
If you're married, just start buying a few pieces here and there and your spouse will have no idea what it cost :)
I run this cannon to 120 PSI without any problems. All parts are rated to 200PSI or more (Air tank 280psi) (Thick Wall PVC Pipe is good stuff), so plenty of safety margin. But I don't plan on going any higher then 130, maybe I'll push it to 140 one day, but probably not.