A low-tech solution to a hi-tech problem.
A while ago I received my new debit card with contactless payment. You don't need a pin number for this contactless payment, just be close to a payment terminal and the money is deducted from your account automatically. To me this looks like a security risk if you ever saw one. First the two-factor authentication (something you have: your debit card and something you know: your pin number) is downgraded to one-factor (only the card). This means anyone who happens to have your card can do a transaction. Second contactless means it works just as well through your wallet or your pocket. Some crooks have already been arrested for making contactless payments by tapping peoples pockets and purse with a portable payment terminal. The banks all know this and lower the risk by only allowing contactless payment for relatively small payments, but this does not fix the underlying problem.
Obviously I immediately disabled contactless payment on all my debit cards. But I have more NFC cards in my wallet. The public transport OV-Chipkaart for example. Their whole system is based on contactless communication so there is no way to disable this. There might even be other cards in my wallet that are NFC capable without me knowing it. The problem with these cards can be narrowed down to the cards being exposed all the time. Even when you are not actively using the card (which is most of the time) your card can still be read or manipulated by anyone close.
To protect all cards when not in use, I made a simple shielded wallet. On AliExpress I bought a square meter of RF shielding fabric. This is thin gray polyester with conductive material weaved in. Unfortunately there is no datasheet available and the specs on the website are very limited. I guess I'll just have to try and see if it works for my application. To test this I wrapped a (fully functional) NFC card in a single layer of shielding fabric and tried it on several card readers. None of the readers registered the card anymore. So the shielding seems to be sufficient for 13.5MHz NFC.
The shielding fabric felt very thin and flimsy so, for a bit more rugged result, I sandwiched the shielding fabric between two layers of bright red canvas. The resulting material both strong and shielding, not to mention Firetruck red. The fabric is then cut to size to fit around two small stacks of 3 credit cards each. The whole thing is then sewed up and you're done.
I tested my new wallet again with several card readers around the city and, just like my tests at home, no reader registered the cards in my wallet. Another advantage of my new wallet is that it's more compact than my previous wallet and does not take up too much of my jeans pocket space.