It’s definitely made by Rothco, because I wanted one and looked it up years ago. You can find the it in Olive Drab and Black, the two colors Jack used for the canvas bag on Amazon.
I bought a smaller version (meant to hold ammunition) of the bag to hold my makeup, haha. So I can vouch for the Olive Drab color being the one for the bag in season 4.
It’s a great bag, by the way! Even though it’s not the same one, it’s super sturdy and tough. I spend a lot of time at Lake Powell and in Mexico and that little bag goes everywhere with me.
How much time would the messenger bag take you? Here is the bag I was thinking of: https://www.amazon.com/Rothco-Classic-Paratrooper-Shoulder-Olive/dp/B000BFJE7O/ref=pd_sim_198_5?_encoding=UTF8&psc=1&refRID=R37VSRFQ5M38DR58ZDKM
What are you interested in? I'd love to do a pair of slippers like this: http://www.knitpicks.com/patterns/Frost_Slippers__D52091220.html
Or if you like scarves or hats, there are all sorts of patterns out there: http://www.leafcutterdesigns.com/blog/scarf-knitting-patterns-best-of-ravelry/
Also all kinds of mittens and gloves: http://www.knitpicks.com/Patterns/MittensGloves_Patterns__L30025203.html