Exercise 2 solution

A figure-eight tied a few finger-widths from the end is a sound choice: its bulge is well wider than a coin-sized hole.
If the hole were twice as wide, the knot may pull through. Options: tie the figure-eight with the cord doubled back on itself, stack a second stopper beside the first, or use a thicker cord.
Test by pulling firmly on the standing part; the knot should stay on the far side of the hole.