We designed, built and tested a new sidebandseparating mixer assembly for the 600-720 GHz band (ALMA Band 9). By concentrating on the input matching and isolation of the quadrature hybrid and associated waveguide components, rather than on the phase and amplitude balance, we minimized standing waves and especially asymmetric reflection paths, which are highly detrimental to the image rejection ratio (IRR). IRRs in excess of 15 dB are obtained repeatably with different blocks and mixer pairs. At the same time, the SSB noise temperature is increased by not more than 20-30K with respect to the bare mixer devices, corresponding to a loss of about 0.5 dB in the waveguide structure. A considerable contribution to the IRR are reflections in the IF system. If these are eliminated, i.e., by using highly matched IF amplifiers, we expect worst-case IRRs of 18 dB or better can be reached, even in array configurations. In less demanding cases, the ample margin in IRR on the RF side can be used to build a system with reasonably matched amplifiers that still meets a typical 10 dB IRR specification. These 2SB mixers are intended for future sideband-separating receivers on the APEX (Chile) and LLAMA (Argentina) observatories and for deployment in any other observatory that would benefit from sideband separation in the 600-720 GHz band.