Millimeter-wave (MMW) glucose concentration estimation possesses a great advantage of non-invasiveness. The long history of investigation, however, has not yet reached practical applications because of its insufficient accuracy and stability. To solve these problems, this paper proposes two high skin-penetration interfaces, which we name equivalent quarter-wavelength interface and equivalent Brewster's-angle interface. We analyze their scattering characteristics in a frequency range of 60 - 90 GHz. Analysis results show that both the interfaces suppress the body-surface scattering, allowing the MMWs to penetrate through body surface into tissues to extract information on blood-glucose concentration with higher sensitivity, e.g., with 147-times enhancement of phase changes. These interfaces can be an important step toward realizing non-invasive blood glucose concentration estimation.