Cutting a slot for a tang in thin metal can be a fiddly experience. When trying to drill a series of holes it is hard to hold down the small thin pieces of metal. Sometimes the drill bit grabs shim and it spins up damaging the it. Milling the slot has similar issues. I have been wrestling with a effective solution for years.
Lately I found a 1 ton hand punch on eBay for $30 that makes quick work of the job. It is like a leather punch on steroids. It can punch different sizes holes in thin brass, spacer material or leather with little effort. It is safe and quick to use and doesn't bend the shims. It comes with seven different sized punches.