My pocket had a great fastener until I needed the pocket to house my new large mobile phone. I used magnetism to fix the issue.
These pockets have been made with two layers - the magnet is now housed, hidden, between these layers.
Step 1: Prep
Start by pulling out the old fastener.
Then I cut a small hole in the edge.
I made sure I had two magnets and wanted to experiment with placement.
I used Hard Disk Drive magnets. I was very careful to never let these magnets get too close to each other because the magnetic force is too strong and will break the magnet up into little pieces. I have done this magnet trick again using significantly weaker rare earth magnets and the quality is very poor. I recommend finding HDD magnets if you need this clasp to be secure. If you tried to pick pocket this pouch/pocket I would feel it because you will be lifting my whole utility belt, not just the flap of this individual pocket.
Step 2: Sewing
This step is very self explanatory.
Pop the magnet in, place it and sew around it.
The trick here is patience. Remember you are sewing a magnet with a metal needle. Breath in, Breath out.
Step 3: Finishing
Do not forget to sew up the original hole. It would be a shame to have the pocket fall apart.
The only con with magnet fasteners that are as strong as this is walking past things like cars, fridges and work benches. Since this pocket sits on my hips I am forever being stuck to things. Bonus is I know that it works!