I've been considering 2nd pack options lately.  I bought a new pack that's a little smaller than my old pack, going from 90L to 75L.  For trips over 7 nights it's a little small.

I tried a 30L barrel, but was optimistic about being able to carry it with my canoe.  Just that much too heavy for me, more the way the weight is all on my shoulders instead of hips.  Too bad really, since a barrel if a very handy item - storage, chair & table all in one.

This looks like it would fit the bill.  Toss on top of my pack tump-like as suggested.