Hey LBF!
If you aren't going for anything fancy, PVC pipe mounted to the cannon and shoulder armor will work just fine for now.
The Plastic pipe is strong enough to hold the weight of the cannon. Make sure that the mount base is secured and tight. Otherwise, the cannon may flop around or slip down your shoulder in the middle of a hunt.
Below is budget design for a cannon mount....
There may be some better ideas than this out there. Take your time to figure out exactly what you need to get your arsenal up and ready.
DangerDavey