Previously I tested out the electron gun and I didn’t see a beam on the phosphor. I wanted to rule out the possibility that there WAS a beam, but it wasn’t hitting the phosphor screen. By moving the screen closer I can see the beam no matter where it is pointed.
So I designed and printed this sweet phosphor holder:
I have a small piece of broken phosphor glass from the CRT. I used the excellent OpenSCAD polygon editor to make a form fitting mount:
In real life:
That was easy!
Prometheus Fusion Perfection 
You still need a ground connection to the phosphor to drain electrons, electrons hitting the phosphor will charge it up as a capacitor plate and it will repell all electrons, no matter what acceleration potential you have.
Steve
Steve, thank you for this comment!
That makes perfect sense, but I never would have thought of it.
How does the plastic behave in vacuum? Has it is made by piling lots of ABS strings, maybe the degassing will prompt torsioning and breakup of the piece??
Old thread but… Are you aware of the need to activate the cathode of a newly-pumped CRT? Atmospheric contamination will prevent electron emission. A pulse of forward g-k current (few tens of mA) with the heater somewhat overdriven is needed to clear this off.