Create an encrypted disk image in Disk Utility about 2.5-10MB in size. Create a password for the disk image, deselect Remember password. Mount the disk image, and drag in your text files containing your PIN numbers. Unmount disk image.
Next time you mount the disk image, it'll ask for your password, with the option of saving the password in a keychain file. Don't save it to your keychain. If you forget the password to your disk image, consider whatever is inside to be lost forever. Use AES128-bit encryption and it should be safe.