You might want to look at the nylon cord sold at fabric stores for blinds. It is typically a bit under 1/8" in diameter and well over 100 pound test. It has a braided sheath and core strands. I carry a 25' hank with me just to have some "rope" with me. The hank weighs less than an ounce. You can get it at Joanne's Fabric or Hancock fabric for a few cents a foot. It is not particularly stretchy and it holds knots very well (but not too well).