A rectilinear job can use a classical "6 point" system as illustrated in the Figure. The incorporation of lugs that simultaneously provide clamping points is helpful. This is an absolutely accurate system. (5 points is not enough, since the part is not uniquely located; 7 points is too many, and will have some points in conflict).

Read more about the how good casting design helps optimize the finishing process in part ten of "Ten Rules for Good Castings"