Along with composite construction comes the responsibility to provide a continuous vent clear through all your layers to the powder charge. This prevents high temp and pressure gas from leaking into the tiny spaces between the layers of your tube. Most people use a plain ole' bolt, stainless or alloy steel to drill the vent in and then install in a drilled and threaded hole in the breech.
Double D had an excellent sketch of just such a "Safety Vent", but I just can't find it tonight. Looks good so far! I know that you are very familiar with a linstock. They are an excellent way to light mortars; you can keep your head away from the muzzle AND vent back blast!
Lets see some cannon building pics, if possible,
Mike and Tracy