For anyone that wants a DIY case I have heard that the metal brackets that house the magnet in a hard drive (HDD) works as a magnetic shield. So if you have an old HDD sitting around (that doesn't work, of course) and have a way to grind the piece flat, that should work if you don't want to return a case or make your own.
EDIT: I found a video of this example:
Thanks! I pulled one of the pci card slot protectors out of the back an old dell case which I notice was the same metal as the HD brackets, and it works well as a shield.