I've made about 150--8 dozen of them for medical purposes so they were 4 layers, pleated style and since elastic in short supply then, I cut up a lot of t-shirts for the ties (I ran out of bias tape about the 4th mask!). I tried using what elastic I had for ear pieces on my trial ones for hubby and self--very hard to wear--guess our ears not located in right place! So most of the ones I make are pleated with t-shirt ties. I paid a teen friend who was saving for a car to make some of the Olsen ones for grandboys---the 12 & 5 yr old loved them cause it made them look like ninjas--the 6yr finds it too confining and the parachute cord bothers him. He's got a bit of a tactile issue. I made 2 layer ones for our family 4th get together--all wore and they weren't too hot.
A friend has made the 3D ones and really likes them so I have my pattern figured out and ready to make--I'm thinking maybe of using elastic (1/8") around the head and another friend makes the elastic adjustable using a pony bead--so I plan to do some experimenting.
If you are making for young kids, I have a panel on backorder from Keepsake that has animal notes, teeth, mustach etc to make the fun--my grandsons thought they'd be great.