I finally found a small plug to attach metal bar stools to. I ordered extra parts in case they can't handle the metal that can go through the plastic. I wish they had a metal insert to prevent wear but hopefully they will hold up.
Pros
These easy to install 0.5" push-on caps provide a permanent solution for many residential, outdoor and marine docking applications including guard rail replacement, steel gate stops, flex bar/rebar. or wrought iron tube caps, threaded bolt security screws or nuts, hub cap cover or any square tube trim.