This pen body was designed to add an interesting geometric flair to an otherwise ordinary pen.
The part was designed to fit the ink refill from the Bic Cristal pens, which can be found here: http://amzn.to/2lndAQm
See the design and build process here:
I printed mine using Simplify3D software 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 placing one of the flat sides down on the bed kept it stable enough during printing.
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.