Instead of using plastic sliders (which would significantly increase the cost of the bag), we devised a way to use a classic lariat knot to hold the bag closed.

The knot uses friction to keep the bag securely closed, yet it is easy to open.

To close the bag, hold the larger lariat knot, and pull on the smaller knot, and the bag will be snugly closed. To open, pull on the big lariat knot while holding the bag.

If the lariat knot gets too loose, you can tighten it by pulling on the tiny short end sticking out of it.