I rough cut the slot on the table saw with a metal cutting blade; three lengthwise passes to make three parallel grooves stopping short of the 11" length. Then I made a beveled crosscut to define the ends. I cut the grooves last bit to the ends with a hacksaw.
A little light work with an angle grinder to cut away the burrs and smooth it a little. But be careful with the grinder, the copper is soft so you need a light touch. Then I hit it with a couple of mill files and finally some steel wool.
There are a few low spots and divots along the way, but cleaned up you really don;t notice them.