Mechanically integrated, magnetically decoupled storage rings were designed for a heavy ion collider for 100 GeV/amu Au, at B= 2.7T. New concepts were developed, including detailed engineering design and cost estimates. A "unicell" contains a half-cell of both rings within a single He vessel. The unicell design is optimized for economical mass production. Survey pads welded to the laminations provide external fiducials to locate the magnet cores. Roller bearing self aligning supports accommodate cool-down shrinkage. The design tolerates relative motion of components resulting from longitudinal shrinkage in the ~15 m long unicell without affecting performance. Magnetic and physical lengths are the same, eliminating waste space. "Achromatic" quadrupoles with sextupoles at both ends are located on a common precision beam tube which aligns and supports a pick-up electrode. The unicell accommodates longer dipoles compared to conventional designs, reducing Bmax, stored energy, and the volume of iron and superconductor. Applications to future machines will be discussed.