Securing Memory Card?
You could use encryption on the entire drive. This can be done with certain apps like EDS (which creates an encrypted container on the device, which requires a password to be opened -- but after that the file system is mounted transparently for all apps to access; these containers are TrueCrypt compatible). Some of these Apps (like e.g. Cryptonite or LUKS Manager) require the device to be rooted.

However, if those encryption does not work on the device filesystem itself (but uses containers), I'm not sure how an app will work with it if it expects files at hard-coded places. You can try and find out, as most of the encryption apps offer free trial versions.