There are instructions on the Wings website for making simmer rings and covers for some of the stoves. I had problems getting these devices to work consistently. Often flames would shoot out the stove bottoms or vents. This simmer can requires virtually no work at all.

Take an empty 6-ounce tomato paste can. Cut it down to 2" in height. Punch a single hole in the can. You're done! This can burns the fuel slowly due to poor air circulation. The pictured simmer can does not have a hole punched into it. I added a single hole later to allow a small amount of airflow from bottom to top.