I would recommend a soft open frameless pack, like a Duluth, which is made of canvas. We can fit three food barrels into our Duluth, but they are olive barrels (watertight) not the standard blue ones. I looked at the 115 L dry bags on the MEC site and it looks like your barrel might just squeeze into one, though paying for a drybag and then putting something rigid and waterproof in it doesn't seem like a great plan to me.
Here are our two bags surrounded by some other stuff for scale:
And here is one alone with the outlines of the barrel ends clearly visible:
I prefer soft frameless bags because frames get caught up in canoes and generally tick me off if I need to climb over them. Especially when you have passengers (my children first went in a canoe at 6 weeks, and my oldest tripped at 11.5 months) a nice soft Duluth to lean back on is essential in-boat furniture. Frames are helpful if you're going to carry 20-40 pounds all day. They don't help much if you're going to carry 90 pounds 700m up and down a hill, and they get in the way all the rest of the time, so I stick with my 30+ year old canvas and leather - they've outlasted all my other camping equipment, btw.
We put our three narrow barrels into the bag sideways, with the mouths always at the same end. If you have two, you could consider putting them in upright which might make it easier to access food without taking the barrels out of the bag.