Cave,
Mine was made of fiberglass. It was like two straps around my chin and one across the forehead. It anchors your head to the table at six spots, and has small lines on it to aid in alignment. They used laser allignment to make sure the machine is aligned properly, and your treatment is highly accurate. During my treatment they also immobilized my arms, looping a line around both wrists and around my feet. The staff monitored me on 3 different cameras, and we had a signal (raising my knees) if I was in trouble. I didn't have any trouble with claustrophobia (21 years in submarines helps), but those who did were offered anti-anxiety meds that helped. In my case, I had to lie there typically for about 20 to 25 minutes. I got to the point that I could actually fall asleep like that.
You'll probably get a "practice" session when they do a simulation and make your first mask. That'll give you some idea of what to look forward to.

Good luck in your treatment. Keep coming back here with your questions. There's an incredible amount of knowledge and support available here.

Good Health

Chuck