To make one you will need a stiff snug fitting case, cutting knife, pen and cutting board. These cases are inexpensive so if you make a mistake its not the end of the world.
Just cut 3 slots into the case. Make sure you draw straight lines inside the case and that the flap is centered. Don't make the flap too narrow as you want some strength. You can make it as wide as you like. You may want to make a template (on card) of the rectangle flap first and then draw around it inside the case. Make sure the flap is not too long or too short. The bottom of the flap should be reasonably close to the edge of the case too.
To open the flap, just flex the plastic by firmly pulling it with your thumb towards the center, or use your fingernail to catch the edge. To close it just click it it back to its original position and it stays in nice and flush. It will not accidently pop open. The flap is virtually invisible when closed.
It works brilliantly. I am well chuffed!