This is irrelevant to cloning NFC-enabled cards. Neither Google Wallet nor ISIS broadcasts payment card NFC data when your phone is locked (or even when it's unlocked--usually you need to enter the app and then enter your PIN first).
What you really should be getting is an RF-shielded wallet for NFC-enabled cards. Your phone doesn't need anything shielding it, and most phones have sane permission models around how you permit apps to use your GPS.
> What you really should be getting is an RF-shielded wallet for NFC-enabled cards.
And when you do, go with something that shields everything in the wallet. (I bought ID Stronghold wallets for myself and the family.)
Here in London it should be possible to market these wallets with an extra twist. Oyster cards are used everywhere, and for the last 4-5 months I've noticed a constant stream of announcements - "Please keep your oyster and contactless payment cards separate to prevent card clash." An enterprising individual with import and retail experience could tap into this market by selling wallets with one outside - unshielded - pocket for the Oyster card, and everything else inside fully shielded.
People in general don't care about privacy or security, but they do care about convenience. So, by way of introducing a convenient way to prevent card clash, they would also get automatic protection against these drive-by NFC payment card attacks.
My experience is that a fully-shielded wallet isn't so bad.
I just got back to the US from a month in Europe. The entire time, I was carrying passport, credit cards, transit-system cards, hotel key cards, etc. in a fully shielded wallet. It wasn't a problem at all to have to pull out the transit card when necessary in order to get on/off a tram or bus, or enter/leave a station (hotel key card has to come out anyway, since often you have to put it in the slot by the door to turn on the room's lights). And the peace of mind is worth it.
(my only actual complaint about the wallet is that I bought it because it had an internal zippered pouch for coins, something that's much more useful for EUR than for USD, but the zipper broke after less than a week)
But I'm not actually concerned about my credit card number getting stolen or being tracked by ne'er do wells with NFC readers. There are much easier ways to steal my credit card or track me. I just want to get through the turnstile without looking like a tourist.
What you really should be getting is an RF-shielded wallet for NFC-enabled cards. Your phone doesn't need anything shielding it, and most phones have sane permission models around how you permit apps to use your GPS.