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.
I built the cap with the open end down, so the entire conical part is a big overhanging roof. This worked OK for me without needing supports, although the interior is a little rough in some spots. I used 20% infill but maybe 10% would work also.
UPDATE: changed files to include a notch for tool to pry off printer bed.