I wanted to improve this joint component for a better fit on rods.
This thing has an internal taper which allows you to slide the rod in straight for a certain distance, then it tapers towards the top for a tighter friction fit.
There is now an opening in the top, so that if you want to fasten your rod into the sleeve, you can easily just poor a bit of epoxy into the opening. The exposure to air should make for a better epoxy cure than if there were no hole.
This one mates perfectly with this other thing: thingiverse.com/thing:8130
The two combined form a fairly tight joint with very little wiggle.