Another alternative is the braided, cored, nylon cord sold for window blinds at your local fabric store. This cord is strong (around 100 lb test), non-stretchy, and has a harder finish than masons cord so is less likely to pick up weed seeds and other debris. It is a bit heavier per foot than masons cord but is a lot easier to untie after a knot has been loaded. The stuff I use weighs just about one oz for 25'. Doesn't reflect light though.
You might need to look around a bit because some of the blind cord I have seen (at WalMart) has a fuzzy, snaggy surface that markedly reduces its usability.