I inherited one of Mike Ameling's customers for these and I've been making them from old (60s)leaf springs. Apparently they are very effective, it's hard for me to tell as my technique with the flint is not that good, so If I can make sparks, they pass the QC test! The striker needs to be dead hard and you need to grind the decarb layer off the striking surface.
This is the best set of instructions I've yet seen:
http://www.angelfire.com/journal2/firefromsteel/
As Mike says at the bottom of the page, occasionally you'll make a dud. If a sample from a piece of steel doesn't work, try a few more from different areas before you chuck it.
If you're making any number of these, you'll soon exhaust your supply of old files. Springs are more plentiful and they don't get as brittle when hardened.