This conical end cap with 5mm central hole fits inside a 2-inch schedule 40 PVC pipe (Home Depot 2 inch conduit). My print requires a firm push to fit it inside the pipe, and after you have done so, the conical exterior is hard to grip, so the cap is difficult to remove except by pressing it out from the other end. SCAD file provided so you can modify to suit.

I'm using it to hold a 5 mm optical part centered in a pipe. Could also form a nozzle, or a pivot point. At the moment the SCAD file preview Thingiverse shows is wrong, maybe they assume the older syntax. I am using OpenSCAD 2014.03

EDIT May 26: Updated SCAD and STL files to fix conical part thickness uniformity.