You're going to need:
* a scrap of soft material such as velvet or fleece (mine's left over from my son's turtle hat
- excellent instructable, that...) to form the brush.
* The end of the ink reservoir from a Bic-style biro
* A twist-tie or length of fine wire (roughly 8-9 cm)
* Some glue
* A small lump of Sugru (less than a sachet; have something lined up to use up the rest of it. Fix that dodgy USB cable or something...)
Okay, blu-tack would probably work as well as Sugru, initially at least, but I like Sugru's permanence and lightness and all-round cool cred. So there.