Bill,
There should be a patient navigator, a medical social worker or an omsbudman where you are being treated. These people can help you with this. Maybe the new RO will be helpful.
For now, I suggest that instead of trying to work on your claustrophobia which could be along process you should concentrate on getting through the mask and radiation. The suggestions offered by those who have been through this issue seem to be very good. Perhaps the best for you would be the nose and mouth holes.
There is a difference between overcoming a phobia and controlling it. I was terrified of fire and still am. In college I made my own therapy which consisted of having the courage to hold a lit cigaret, putting one in my mouth after a friend lit it, and finally lighting my own. This was not an over night process and now I can actually strike a match but am still shakey when one begins to burn close to my fingers. I get that physical internal shaking even when someone else is letting a match or cigarettte burn down to what I feel is a "dangerous level." I have found what I think is the source of this irrational fear but that is irrelevant to the fact that I will probably always have this phobia.

Another thought: Why don't you make your own sign(s) with some pictures such Munch's "The Scream," Darth Vader, Houdini ,... and/or some slogans or phrases that you may think would help.
Good luck,
Malka