While you’re lightmap UVs may need a little padding between the UV islands there is nothing wrong with this mesh.
The wavy low quality shadow you see on the ground is the result of a low Lightmap Resolution for the ground plane. Not the box.
The lightmap does not handle how shadows are cast and baked onto other meshes, but how it bakes a texture for itself.
You should take a look at our Wiki Lighting Troubleshooting and Tips guide for some simple demonstrations to help explain things such as this. 
You can find the link in my signature below.