[Greysard]

Cards with magnetic stripe cannot be read remotely. Only RFID cards can, but I haven't even seen those. Smart cards with contacts are not affected at all (they are safe.)

Mylar is not affecting EM fields; it's a good insulator, but that doesn't help here. You need a conductive shell around protected items. No gaps allowed. Tongue and groove is the best.

[gfbtbb]

The person that recommended Mylar should have expanded a tad and said “metal coated Mylar”. Aluminum works well.