This is the simplest form, it's a tube with one end capped. I fussed with this until I could get it watertight. I use the reservoir ring that comes with a rapidograph pen and superglue that to the end of the 3D printed part. This increases the ink I can hold from about 1ml to about 6ml for long plotted drawings. Note: superglue seems to work best, I used a silicon glue and that did not work.