"I plan to put my canister inside my pack because there is so much volume. If it doesn't fit right I will go ahead and put it on top with the Y strap."
Good plan --- putting it *inside* the pack.
I carried a bear can with a 2006 version of the Mariposa Plus through the Sierras last year. I started out with it strapped externally, under the Y strap. I knew ahead of time that a slippery bear can wouldn't be very secure under the Y strap so I trimmed down an old Army sleeping bag wrap unit that I strapped to the can to give something more solid for the Y strap to connect to.
I did this for the first couple or three days but it became too much of a PITA, the can was always trying to slide off to the side. Even though the Y strap stayed with the can, or vice versa, the total weight (2+ pounds of food per day by that point in the trip) was such that the can just had a mind of its own.
I went out to Independence via Kearsarge Pass and mailed home some stuff I found I didn't need, strapped more stuff to the outside of the pack, and managed just fine with the bear can inside the pack from then on. Nice to have the volume to do that!