Interesting idea...odds are it works pretty good, too!
Here's an easier way to defeat the thieves: Take some copper tape and line the inside of the cash pouch with it. Once you close your wallet that copper tape "shield" will be keeping any RF emissions from getting into or out of your wallet. You'll never know the tape's there, as it's thin, but a good RF shield material. Cheaper than $20 too..
ISO
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The Essence of Compassion
"Resolve to be tender with the young, Compassionate with the aged,
Sympathetic with the striving and tolerant of the weak and wrong......
...... because sometime in your life you have been guilty of all of these."