Polarfleece at walmart in the fabric section.
Cut panels the size of each side of the pack plus about six inches on all sides. For instance, if the back panel is 12 X 20 inches, cut the piece for that side 24 X 32. Do the same for all the other sides.
Now, take some good scissors and "fringe" the edges in about 5 1/2" on each side. About 1/2" wide fringes work best. When you have all the side panels cut and fringed, start with one edge and begin tying the fringes together. That is, the front panel fringes will be tied to the side panel fringes. Tie 'em tight with a good overhand knot. You are basically making a pack cover without sewing anything at all. The end result will be a fleece cover that will have nice fringes at the seams all the way around. I made a fletch cover this way for my quiver and it worked real nice.
p.s. In a pinch, you can cut off a fringe and use it for a scent wick when hunting. Been there, done that.