What size end mill bit are you going to be using. There are some cheap movable tables that can clamp or bolt to the drill press work surface that can give you travel in an X and Y direction if that helps. If your just going to be doing plunge cuts, you can cut a piece of plywood and place you metal on top of it and clamp both of them to the drill press surface. That will help keep the metal from moving as your milling it and once you break thru it, the cutter will hit the soft plywood and not mess up the drill press work surface.
Plunge cuts are what was in my mind but now that you mention it one of those tables would be pretty cool.
