Yourtube video on how to pack a monitor. I'm not convinced this will work for a vintage CRT. Pressure would be applied to the neck as the foam expands and being contained in the pack it couldn't flow, especially if the face of the CRT is being held in place. A poured foam wouldn't have that problem. The seal between the glass and the metal on the composite CRT is the critical issue. I wouldn't want those areas to be put under any unnecessary pressure. Even a CRT like a 12LP4 is fragile where the neck expands.
http://www.youtube.com/watch?v=F4YDzwEA3v0&NR=1