Dear Blue Grouse;
Please allow an old man nearly 80 to provide you with some advice on the selection of the steel (which should be spring steel or something that can be hardened) basis to to use to minimize the forming of the part work.
Measure the thickness of the steel with precision( I have found in Stephen Grant ejectors trippers that they are not uniform thickness front to rear) then drive over to the home supply store or second hand tool store and measure the thickness of the cheap carpenter hand saws (I have bought them for $5.00 or so in the past) finding one that meets your thickness needs. Even the cheap saws are made of good spring steel--especially the cheap Buck brand. This will save you a lot of filing work. While you are there buy a round chain saw file for the rounding of the edge of the slot you are going to make.
