I present my Quarter Inch Hex Bit Pen, designed to be used in hex bit holders or extenders, as a modular keychain pen. It can however be used on it's own, for short messages and notes, without discomfort.
To complete the assembly, a Cross Matrix ballpoint refill is required, as well as an M3x3mm grub screw (set screw), which retains the refill. These items are not included.
The back portion of the pen will need an M3 thread tapped into it, for the grub screw.
The central hole of the pen may also need to be drilled out slightly, in case the printed pen body does not end up being completely straight. When drilling, make sure not to drill all the way through the centre and widen the diameter at the tip, as this narrowed point is what the refill is held against.
The outer faces may need a small amount of sanding, to ensure a correct fit with the specific bit holder chosen.