No, I am not mistaken in the title, this is a pencil case!
No need to search for the opening, you can open it anywhere.
Supplies : 9 non-separable zippers
(done with18 cm long zippers but you can choose your size)
Put the zippers side by side, overlapping slightly (if you look closely the edge, there is a 3 mm border, I used it as a guide).
To be sure it doesn't move during sewing, I first overcast the zippers (if they aren't straight the case may take a "spiral" effect that is not very nice).
The longest is changing thread and can colors (I'm a perfectionist, I know ...)..
Don't forget to move the zipper along the way not to interfere with the presser foot.
Zigzag sewing the ends.
Sew the last two zippers to form the tube.
Well, then I'm sorry but we need to do math !!
You learned (yes, yes, it was part of the program):
C = πD (or C = π x 2R)
Clearly, the circumference is equal to π x diameter (or radius 2x x π)
Measure the case circumference and cut circles = π / (C + seam allowance)
(WARNING seam allowance is added to the circumference).
Cut 2 circles in a matching cotton and 2 circles in a stronger fabric (I took a blackout fabric).
I remind you that children always lost pen caps, using a strong fabric prevents from pen holes...
Overlay a cotton circle and a circle of strong fabric and sew zigzag.
Pin a circle on the tube zipper (right sides facing) and sew a straight stitch.
Well here I admit that it is sometimes a bit of a chore not to make fold.
I sew once, I check that everything is well placed and I double (even triple) the seam.
Do the same on the other side (don't forget to open a zipper in order to return the case!!)