I checked the website of the American-Swedish Historical Society; it doesn't mention the Monitor drawing. (I was hoping those folks might sell prints of it in their gift shop. No luck.) My information on the location of that drawing is now more than twenty years old; for all I know it may be someplace else now.
philp's logic regarding the top of the turret makes sense. The plating arrangement would have one other big virtue: it would keep water out in a rough seaway - or, for that matter, a rain storm. (The Monitor was a miserable seaboat; that's one reason why she sank.) On the other hand, the iron grating shown in some reconstructions has small enough openings that a man could stand on it with no trouble. (If I remember correctly, somebody's standing on top of the turret in one of the extant photographs.) And imagine what the temperature, visibility, and atmosphere inside that turret must have been like when the guns had been firing for a while - unless there was some way for the smoke to get out.
I can see arguments both ways on this point. Let's hope we live long enough to see the definitive answer revealed. I haven't been keeping up with the story of how the turret is being conserved, but I believe it's now submerged in a tank of chemicals - and still upside down. The conservation process in cases like this usually takes years; I suspect the entire Monitor turret project will take decades, even if the necessary funding continues to be available.
Youth, talent, hard work, and enthusiasm are no match for old age and treachery.