I decided to have a go at a pen body, since the shell for my favorite pen recently broke. I used a band saw, spindle sander, and belt sander - I made it at TechShop http://techshop.ws
I started with a small piece of wood, in which I drilled a hole of the diameter of the fitted end of my pen's innards.
From there, I held it up to a 1" spindle sander on each of the four sides, which created some nice grooves but also very sharp edges.
I then used belt sander to shape it a little better, angling the front to better transition to the pen's plastic, and angling the back to better "fade out."
I then held the body and marked where my forefinger and thumb most naturally placed on it - I sanded those areas with the spindle sander.
The end result had some very sharp and uncomfortable corners, which I softened by hand.
And there you have it! An easy, cheap, customizable, and comfortable pen body for all your pen-based needs.
I also included a photo of another pen body I made - although I used generic scrap wood for this, I think this particular technique would be great with wood that still has its bark - the color/texture variation would look good, though I am unsure of its potential comfort.