I started with a hunk of steel as wide as the top of the lock, then drew the part onto it.
Drill out where the string will rest. I used a bit a little larger than the string diameter.
Now the "fun" starts, hack away with the saw. Stay outside the lines. It took me about 30 minutes to get everything cut out with a new blade.
Then file it flat.
I should add that the slot where the string goes should be angled down, not horizontal like I did it. In effect because of this mistake I wasted the 300 or so calories it took to gnaw through the steel. It might work...
Also relief cuts are your friends, you can see where tha angle of the cut changes, that is where the cuts were made.