I used jsc's bottle neck to create a hex head plug for my Sawyer Squeeze water filter. This plug covers the dirty water inlet to the filter and helps to keep debris out and retain the rubber washer.

Creality

Ender 3 Pro

No

No

0.1

25%

I printed this part with a 0.1 mm layer height in an attempt to make the part as strong and smooth as possible.

I imported the pco-1881.stl file into a new part in FreeCAD and added a 6-sided regular polygon and extruded it 0.3". Rotation by 90 degrees aligned the hexagon along the axis of the bottle neck. Translating 0.2" along the axis of the bottle neck brought the parts together. Selecting both parts and exporting as a new STL file produced the finished part. Rotate the part by 90 degrees in your slicer when you generate your GCODE file so that the part sits flat on the print bed.