I own a Tred Barta leather arm guard with a round elastic cord. The cord is pretty worn and needed replacing. I stopped at a local fabric store to look for replacement cord, and ended up buying two relatively inexpensive options.
One option was a round cord, one was flat cord. I tried the flat cord first on my arm guard and see no reason to try the round cord. The flat cord is much more comfortable than the original round cord. It does not bite into my arm nearly as much as the original round cord. You may want to try for yourself.