Recent work on some issues within the Unofficial Patch project (e.g. constructing the new supports for the Windhelm piers) has shown that it is still a problem to add appropriate collision meshes to Skyrim's static objects.
It may have gone unnoticed that there is a new tool, which converts any conventional mesh into a bhkCompressedMeshShape block and adds this to a nif of your liking:
http://niftools.sourceforge.net/wiki/Skyrim/Adding_Collision_Mesh_using_ChunkMerge