Could you be more specific? Which bear canister?

A Garcia or BV500.

A BV500 will not really fit into the pack horizontally. You can force it, and barely make it fit into the very top of the rolltop, but it puts a lot of stress on the fabric and stitching. A Garcia fits well, with a little tension on the fabric. A better option for the BV is to close the rolltop with everything you need in the pack, place the BV on top and use the top straps to hold the BV on top. It works very well, and actually is a more comfortable setup than having the bear can inside the pack because the tension between the top of the rolltop and the top straps will pull the thin backpanel straight, and create more structure to the pack.