eager beaver hunter is what i'm gonna call you, "Tell" you how to make a good cannon? use some reference images and some clay, get your self some model kits to "bash"
sprinkle some imagination into the mix and there you go.... some effort is the key
Wow nibs never would have known that by looking at it. I'm making one the kid next door. This year Halloween is going as the predator. I went to the dollar store or the 99 cent store and got a gun that had almost the same look as the P2 cannon. All I really had to do is take of the handle and the trigger gaurd and add some putty. Took a old goose neck lamp for the arm and some foam to make the mount.
But you can use fun foam to make a cannon like I've seen on the lair in a post.
Mine's actually made from a plastic mineral water bottle with a few latex, plastic and card bits and pieces added (see pic in suit thread). One advantage of this method is weight. The cannon is so light you forget it's there.