Jim,
One way would be to just set the base of the antenna on top of the
pipe and feed the feed line down the pipe. Notch an appropriately
sized cap for that pipe for the radials and vertical element. Set cap
on/over the antenna. Fix the cap to the pipe how ever you want to,
glue?
I agree about the unstableness of the PVC pipe., it do bend and
sway if it's very long. So, use a shorter section on top of something
else? Clamp to a vent pipe on top of the house?
Have fun...
- 'Doc