hey, you try and bend an electron traveling at the speed of light 110 degrees at the neck though the envelope and then hit the same spot in a straight line a half millimeter apart. wouldn't be so bad if the turn wasn't so sharp...but then you tube would be 40 inches long.
superflat tube geometry requires some serious processing power and tuning, since you basically have to rely on a microcontroller and dedicated chips to automatically change the scan timing/duration 28,800 times a second (every line, 60 times a second.) and that's just the horizontal sweep.
it's not going to be perfect, you are going have to live with some compromises in your image unless you shell out for some serious video display monitors. the linearity is going to be a little uneven. The corners are going to be a little weird. The focus is going to be a little off here or there. it's a CRT, they aren't perfect. That's the charm of them.
you want a insanely bright, perfectly square, perfectly in focus, perfectly linear, perfectly stable picture 100% of the time, buy an LCD.
super bright, super sharp, HD displays have changed your idea of what "good" looks like.
Throw a game on there with stuff moving around, you won't notice the corner convergence is off cause literally nothing gets displayed there. play a game on there. you won't have time to notice the linearity is slightly off cause you are too busy getting killed by enemies.