So, this job has been worrying me for months. The tailpost is a key structural part of the aircraft, and it needs to be right so the aircraft flies straight. Also, you need to notch out the four longeron tubes that come together at the back of the aircraft to accept the tailpost tube. Don't notch them too much or they are screwed. Don't notch them in the centre, the tailpost leans.
Turns out it wasn't all that hard. I just spent a lot of time looking at it and thinking. I used those fancy new EAA clamps to do the job, too.
The tailpost is secured to the jig table top and to the ceiling, so unless the house moves, it should stay plumb.