According to everything I've read, the RFID chip in a passport card contains no PII, only a unique identifying number. So personally I've never taken any extraordinary precaution with my passport card.

However, I do routinely use an aluminum wallet/card case which blocks RFID signals from my various ID badges. If I were concerned about protecting my passport card, I'd keep it in my aluminum wallet, along with my various other IDs and cards. I like those wallets mostly for the convenience, small format, and easy access to the cards, but the RFID blocking is a side benefit.