What makes it tough is that, with all the pots and pans all over the place, it's difficult to see what the backpack really looks like.
Probably the easiest thing to do would be to make a simple bag (front, back, narrow bottom and 2 narrow sides) with two shoulder straps and a flap that folds over the top. From the pictures, there are a few small pouches sewn on the outside - if you opt to include these, it would probably be easiest to sew these onto the front/back/sides first, before assembling the entire main bag. Make sure to include two long straps at the top for tying the bedroll on.
You try looking through the free patterns offered here to get some construction methods/ideas, or to find a pttern you could modify:
Pictures and tutorials from my costuming portfolio at Syagria's Cosplay Site!
Star Trek Original Series Uniform!
Original Creation for AB13
Most Recent Con:
PAX East, Anime Boston 2013