I machined the ball ends so they would be a nice press fit into the tubes.
Then I put in 2 self tapping screw in each side through the wall of the pipe to secure them. Much better than star nuts!
Anyway without access to a lathe this info is useless to you
You could easily do it with a linisher/belt sander also, the plastic is easily ground, however take your time and continually measure them before taking more material off than you need to.