In the film Frodo's backpack seems to be on the large side, covering most of the character's back. The cloth of the backpack is a pinkish brown, darker than the frock coat, and the pattern on it is dark brown or brown-black. The leather straps, top flap closure, front flap closure, and side pockets all appear to be smooth medium-brown leather. The front pocket is a tan suede. The closures seem to be loops of leather lacing that hook around barrel-shaped buttons.
There is a pretty good photo of Frodo's backpack in the costume-breakdown section of the extended DVD set which we do not have a link for it.
There are close commercial patterns but they run on the small side.
To avoid cutting a slit/slits in your cloak for the backpack straps to go through, do not attach the lower ends of the straps to the backpack. Or alternatively make the bottom straps removable from the bottom of the bag.
With a cloak, just tie them together behind your back under your cloak. Adding a snap closure or hook-and-eye closure would work even better, be easier to do and undo, and would eliminate the lumpy knot.
Pattern may have been woven in, but can easily be stamped in a well.
Stamps can be quick carved out of anything from baked potatoes to sponge to plastic.