Like this, but you you could cut an opening on the front.
http://4.bp.blogspot.com/_kMSYwo-WCBc/SwKhYMH7aPI/AAAAAAAABJE/BZns2pAgCM0/s1600/Ground+Nesting+Boxes.JPG
Also my ducks don't use boxes they make a nest out of straw on the ground.
Golfballs work as fake eggs. You can either make a makeshift nest on the ground with the straw or get a box or plastic container and cut out a door/opening in the front and put straw and the fake eggs in it. My runner lays her eggs where ever she wants in the coop. all spread out. My mallard...