Attached Link: http://linuxcommando.blogspot.com/2008/06/create-file-of-given-size-with-random.html

The best way of doing this would be to use either RTEncoder or OBJAction mover, with a key file that will never be public, e.g. a noise filled image that you delete after encoding.

It would also be a trivial exercise to write a program to XOR a file against random data.

Or if you have access to a Linux machine (and possibly a Mac), see the attached link for how to create a file with random content via the command line.