I’ve had tank valves open rattling around in the trunk and have friends whose tank valves got deformed (“mushroomed”) sliding around in truck beds. Metal plugs solve both problems but I do suggest brass chrome-plates ones (others are stainless) for salt water and relief-valve versions for O2 bottles. Never had a hard time removing a stuck unvented plug with a 1” wrench but if you don’t have one you’re SOL.