I could fabricate an "X" from gold wire and solder it to the back
of the tube, and solder a post perpendicular to that. I don't know
that just soldering a post to the rim of the tube would be very
To get the post centered in the back of the tube, you don't need an
X shape. That's overkill. file or bur a notch across the back of the
tube so you can lay a somewhat heavy wire in the notch that crosses
the center of the back of the tube, and is at least half it's
diameter recessed into the rims of the tube, preferably a bit more.
The guage of that wire should be a bit heavier than your earposts.
Solder the two contact points, but leave the ends projecting a bit out
to the sides for now. Sand or file the back flat again, so the wire
now looks like a flat strap crossing the back of the tube. Solder
your earpost in the center. Now you can trim the sides of the wire
flush with the side of the tube. If you want to hang something from
the tube itself, just solder a little jump ring, or a 3/4 jump ring,
to the side of the tube...