Does the kayak have sealed bulkheads? I know folks who use their kayak hatches for food (and sometimes water) critter protection, and who for that very reason do not drill a tiny air pressure vent hole in the bulkhead.
I occasionally carry excess/starter foods in soft containers or dry bags, but I eat through that first and hang it in camp; I wouldn’t want to dispute ownership with a bear that was grubbing under the decks of my Monarch.
A 30L or even 60L blue barrel will fit behind the seat in the Monarch, although the 60 leaves zero room for trim adjustment. The 30L allows for about a foot of fore or aft adjustment.
I use a couple of these squat (_) barrels in lieu of the 30L on no-portage trips.
http://s1285.photobucket.com/user/CooperMcCrea/media/January%20Shop%20Days/P1250589.jpg.html?sort=2&o=3
Those fit under the decks much better than a \_/ shaped screw-top bucket and allow for easier trim adjustment, especially on trips where I’m carrying all of my potable water and give that weight priority placement near the center of the hull.
The Monarch is pretty benign in most wind and wave conditions, but it is trim sensitive in following seas and the ability to shift gear weight without repacking the entire hull is helpful.