In my August bag, the Swim & Gym bag, I've come up with two methods for the outer mesh pockets, one with elastic threaded through a casing, and the other with fold over elastic along the top.
I've done a little Youtube video to show you how to add the fold over elastic.
It's a little trickier, but if you like a challenge, why not give it a go?
Frequently Asked Questions:
What type of elastic should I use?
Fold over Elastic
How long should the elastic be?
Cut the elastic unstretched to the length you'd like the pocket top to be