I just noticed that Staedtler let’s you design your own pen in 3D. They will then 3D-print it and send it to you.
What a great idea.
As far as I can tell this service is currently only available in Germany and you need an iPad to design the pen.
PS: For Staedtler’s web sites you can’t leave out the “www.”, so staedtler.de will (currently) not work. You can find the page for this pen at https://www.staedtler.de/de/produkte/staedtler-3dsigner/.
I believe that the use of the image shown, taken from Staedtler’s web site, in this blog post falls under “fair dealing” as described by the UK Copyright service.