These torpedo roof vents have been very nicely cast for us in brass.
They are sold individually and are the correct scale for 16mm and G scale.
 
Depending on how many are bought, the vents can come as part of a sprue or basically cut from the sprue. If they come on the sprue, they will require cutting off with a fine 32 TPI tooth hacksaw. All will need a basic filing on the bottom of the mounting shaft to finish.
Max width of oval = 9.5mm
Max height from base to top = 7.5mm
Diameter of mounting shaft = 4mm
Approx length of mounting shaft = 5mm