Don't cut the slot full-width in one pass. Use an end mill smaller than the desired slot width. Start right down the middle of the slot, and take cuts running back and forth the length of the slot, going deeper by about 1/2 the diameter of the cutter each pass, until you break through the bottom. Then, offset the part a bit, and take one side up to the desired slot dimension. Then, offset the other way and do the other side. if the machine is tight, do these cuts in the climb-milling direction, where the feed is with the rotation of the cutting edge. This gives dramatically better finish in aluminum. (If the machine is loose, this cutting direction can cause the work to jump into the cut, so not a good idea.)
This is all MUCH easier with a CNC machine, of course.
Jon