Notes:
I printed mine using Simplify3D s...oftware and my Dremel Idea Builder, using PLA filament and printing in High detail with 100% infill (solid). The software will likely try to add support material on the inside where the ink will go. This would be too hard to remove and you can get by without it. Instead, I used manually-added supports on each end, and along the outside to keep the pen from rolling as it was being printed.
How I Designed This
The pen was designed in Fusion 360. First the ink refill was measured and modeled, and then the pen body was modeled around it. Before making the larger model, I created a few "pen tip" test pieces with different hole diameters to find the best size hole for the ink to fit in.