The headset cap is the small piece at the top of the fork. It keeps dust and dirt from entering the headset assembly. It is a relatively simple part and be easily customized. There are many websites and stores were they could be bought but, if you have the ability to print you're own or CNC it then why not. You can put almost any design onto it, or you can even incorporate a Go-Pro mount (However I'm sure thats patented so I'm not going to post how to copy it). I am new to 3D design so feel free to improve on my part and please comment a pic if you do. I currently don't have access to a 3D printer (However if I'm lucky that could change) so I haven't printed this part (yet).
To Design this you'll need:
A 3D design program. I used Autodesk 3Ds Max (because I'm a student :), There are free alternatives such as Google sketchup for those who aren't students. I don't know how to use that software but megaduty has posted many sketchup instructables and was nice enough to help me start using the program.
Thats all you need to design it.
Helpful items, A bike, A Ruler.