Egg rings work very well for making breakfast sandwiches, but why spend money on commercial egg rings if you already have jars with lids?
Household weblog The H Blog shares that a standard 280g wide-mouth pressed steel jar ring will hold a large egg perfectly. Just spray the ring with your cooking spray of choice. Place the ring in your skillet and crack an egg inside. Cook on medium low heat for a minute or two and then remove the ring; the egg will stay in the circular shape. Flip the egg over and cook for another minute and it'll be ready to eat.
You'll find that this circular egg will fit a bagel or english muffin perfectly.