I'm happy to report that the first Rhino gas mask is complete. It took me just under one month to construct from start to finish. Here then are the photos.
Since 73% of respondents to my poll on horn direction favored up, I have gone with that.
This first mask turned out smaller than I had expected. Definitely a size small. I have listed it on Ebay with a $1 starting bid. There are more photos there. Rhino on Ebay