Bring the sugar and 2 cups water to a boil in a heavy saucepan. When the liquid is clear and bubbling, reduce the heat to low. Add the lemon slices and simmer until the rinds are translucent, 30 to 35 minutes. Transfer the wheels to baking sheets lined with parchment paper, and let cool completely. Extra wheels can be stored in a container with a tight-fitting lid for up to 1 month in the cupboard.