I saw the pictures but one fellow put a small plastic tool box attatched with "Zip ties" It is lockable and weather proof. I think the total cost was $10! He just drilled some holes inside the bottom of the tool box, lining it up with the rack. I would use U-bolts through for strength, voilla a lockable inexpensive trunk for your bike. You have to put a padlock on it of course. Personally I prefer a backpack because I can just bring it with me.
Here is the link (hope it works)
http://www.instructables.com/id/E2V3Y9E9J7EP2868DU/