I have a canvas winter cover for my boat. I didn't by it; it came with the boat when I bought it. I did have it re-stitched, though.
There are grommets around the bottom edge which get tied to the perforated rail at intervals. I use the cheap, hardware store cord as pictured because it is soft and seems to take a pretty good knot unlike high class marine line that's too hard to knot easily. It chafes on the sharp edges of the rail, of course, and many ties only last one year. That's fine.
The problem is that even though the knots look nice and tight, they come loose as the tent flaps in the breeze. I use square knots because they are easy, but I'm thinking I need a better knot. Any suggestions?