90 degree cavosurface margins, axial wall following the contour of the tooth, flat pulpal and gingival floor, converging buccal and lingual walls, broken contact on buccal/lingual/gingival walls of the proximal box. add retentive grooves if needed.
Include a reverse S curve to the buccal by the proximal box and a dovetail on the occlusal portion of the preparation that does not extend over the marginal ridge.