Why canvas? The reason artists went to canvas instead of wood panels as a substrate for oil painting is that larger sizes can be accomodated, and so that the paintings can be rolled up for storage and transport.
But with the advent of modern plywoods, it's much easier to span large surfaces with wood than before without risking cracks, and of course you're not going to be rolling these things up with shells glued on.
So why not make up a plywood panel, and forget about the canvas? It will be a lot easier to glue to without fear of shedding shells everywhere. You can use a range of glues, but since you're likely to have some gaps to fill, try the thicker-bodied ones that dispense with caulking guns, like construction adhesive or silicone.