Theater often requires the audience to suspend their disbelief during the telling of the story so they can become emotionally involved in the play. It is easier for the audience to do this if you have props and sets that leave less to the imagination. Pantomiming a firearm, for example, is more distracting to an audience than using a toy gun that they can see for themselves. Creating makeshift props can therefore greatly enhance the story you're trying to tell. If you need a cannon in your show, it is more cost-effective to make your own prop version.
Paint a long piece of PVC piping black. You can get them in various widths and lengths from a hardware store. The wider the PVC pipe, the more impressive your cannon will be. Place newspaper or a plastic drop cloth under the piping to prevent paint from spreading.
Cut a hole with scissors in the top side of a refrigerator box and a semicircle on the front panel that is large enough for the PVC pipe to fit into. The hole should be near the end of the box and the two holes should be connected so the pipe slides in and rests against the semicircle.
Apply brown paint to the refrigerator box to give it a more wooden appearance from a distance.
Slide the PVC pipe into the refrigerator box once they both have dried. You may need to add some weight to the lower end of the PVC pipe to keep it from tipping. Secure rocks or other weighted material to the end of the pipe with duct tape as needed to keep the barrel stable.
Rest the makeshift cannon onto a flat board with wheels so you can move it on and off stage more quickly.