What if you tie a double (or triple) overhand knot with a similar line around the main suspension line at each corner? The holes in the wood will need to closely match the diameter of the main suspension line so the shelf will sits on the added thickness of the double overhand knot.
Braided cotton, as well as twisted cotton, will have creep over time, and the length will also change with the weather, so what KC said about pre-stretching the line is a good idea. It won't eliminate all creep, but it will produce a better product.
Pat