hey northumbriman, heres an idea for ya, on the knife, at the end of the handle, you could make a rounded notch in it
a bit like this
https://www.dropbox.com/s/xc07pqc26gof4u...0notch.png
to be used as something to strike your firestarter with, this would have a few advantages over the bits of metal provided with them usually, or with using either the knife's blade (which would ruin it) or the back of the knife, these advantages being, you could aim the sparks with a far greater degree of accuracy, and it also makes striking your firestarter easier
and if you got one of those magnesium block firestarters, it should do a nice job of scraping off bits of magnesium too
anyway, just an idea for ya.
on a side note, how custom are you willing to go and what sort of steels are you happy using? i got a few ideas in mind