Mesh Tools complained about the original not being watertight, so I tossed this into the STL Repair tool from FormWare (output follows).

No changes were made other than repairing the STL.

• -> Uploading..FloorCableCover3_v6.stl
• -> In queue..
• -> Assigned to repair node
• -> Transfering to repair node
• -> Transfer done.
• -> Analyzing file
• --> 0 Naked edges (?)
• --> 0 Planar holes (?)
• --> 0 Non-Planar holes (?)
• --> 4 Non-Manifold edges (?)
• --> 5 Inverted normals (?)
• --> 0 Duplicate faces (?)
• --> 0 Degenerate faces (?)
• --> 0 Disjoint shells (?)
• -> Repairing: 100.00%
• -> Converting back
• -> File repaired
• Time needed for repair (ms): 939
• --> Vertex count changed from 1369 to 1575 (+206)
• --> Triangle count changed from 2730 to 3146 (+416)