We report a nanofabrication method that combines block copolymer directed assembly with e-beam lithography to achieve highly uniform rectangular patterns with a critical dimension of 16 nm, a full pitch of 27 nm, and arbitrary aspect ratio. This fabrication method enables geometries that are not natural to block copolymer assembly, preserves both the feature uniformity and the center-to-center spacing of the original block copolymer, sustains long-range translational order, and facilitates high-resolution, high-density patterns through feature density multiplication. These highly uniform arrays of dense rectangular features are particularly attractive for fabricating magnetic bit patterned media with high bit aspect ratio.