We cut a nearly bore-sized circle from plywood using a hole-saw. This disc must overlap the PVC pipe completely (be same or a bit larger diameter.) Plywood used in this manner by us is 1/2 inch for small bore cannons and 3/4 for say, 3" bore guns.
We did this to make canister rounds which did not need to stay together after firing, and in that case the PVC pipe section can then be attached to the face of the wood disc however you like, only needs to be strong enough to keep it together while loading etc. since G forces will keep it together after firing it, at least while it is in the bore. We plugged the pilot hole in the center of the disc (that's left from most hole saws), with a short bolt, bolt head toward the rear of the projectile of course, that way no nut is needed on the front side if the bolt fits tightly.
If you want both discs to stay attached to PVC you need to put a bit of thought into that, perhaps a wood disc on either end of PVC, attached by epoxy if you like, and a carriage bolt (or threaded rod) thru both discs with head on one outside end of disc and nut and washer on outside of disc on threaded end, or something. When we used the bolting method, we never used epoxy as the bolt when tightened held it all together well enough. We usually used sand for a filler, probably good enough for small guns or mortars and light blackpowder charges.
You may want to put some slots or vents in the front disc so powder gas behind it can blow by; you don't want it to seal well enough so pressure in the bore can pressurize your projectile internally which could cause it to pop open outside the muzzle. The rear disc should fit the bore well but still be easy to load.
Remember the "hole size" is how hole saw sets are labelled, not by the disc size, so maybe a 2 1/8-inch hole saw will leave a disc that's about 2 inches, and if too tight for your bore, you can mount a short stack of them on some rotary tool and sand them down a bit.