I use this one quite a bit:
http://www.burley.com/products/trail...?p=Flatbed&i=7
The hitch will allow the bike to lay flat to either side.
To lock it I disconnect it from the bike, run a cable through both wheels and the frame and lock it to my bike lock. Securing the contents of the trailer is more difficult. I have considered making a thin plywood bed and mounting that on the frame without drilling holes in the frame. Then bolt a lockable box to the bed.
Here's a pic of what it looks like behind my road bike:
