I am sure you are familiar with them. The flat black (sometimes otherwise coated) wrenches that come with a lot of power tools for changing blades, bits, discs, etc. They look like they are stamped out of sheet. I am certainly not going to make a stamping die for one wrench, but I am sure I could cut one out of flat stock on the mill when I need one and a regular mechanics wrench won't fit. The thing is I don't know what steel to use.

How about an inexpensive alloy that might be easily heat treatable. I don't think surface hardening would help for a wrench or a spanner as the cross section would still be softer, but maybe somebody who knows better could speak up?