I agree with bushveld about all "spring steels" not being equal. I feel you would be better off modifying a good quality screwdriver bit by slowly grinding it to the required shape without over-heating it and drawing the temper. If you temper too soft, the bit may twist and deform and damage the screw slot. If you temper it too hard and brittle, it may suddenly break, and cause you to gouge or scratch the surface surrounding your screw. A thin screw slot requires a thin well fitted blade which is more easily broken, and increases the chances of bad things happening. So start with good penetrating oil to help things get moving.
You might also check if Brownell's or Wheeler has magnetic tip individual bits that are close to your needs. My own set of Wheeler bits seem to be a little on the brittle side, and I have had some break on stubborn screws.
Those Crown Cabinet screwdrivers from Amazon do not appear to be hollow ground, and if so, would be unsuitable for most gun work without modification.
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