Resonant cavity-assisted enhancement of optical absorption was a photodetector designing concept emerging about two and half decades ago that responded to the challenge of thinning the photoactive layer while outperforming the efficiency of the monolithic photodetector. However, for many relevant materials, meeting that challenge with such a design unrealistically requires many layer deposition steps, so that the efficiency at goal hardly becomes attainable because of inevitable fabrication faults. Under this circumstance, we suggest a new approach for designing photodetectors with an absorber layer as thin as those in respective resonant cavity-enhanced ones, but concurrently, the overall detector thickness would be much thinner and top-performing. The proposed structures also contain the cavity-absorber arrangement but enclose the cavity by two dielectric one-dimensional grating-on-layer structures with the same grating pitch, instead of the distributed Bragg reflectors typical of the resonant cavity enhancement approach. By a design based on in-house software, the theoretical feasibility of such ∼7.0-8.5 μm thick structures with ∼100% efficiency for a linearly polarized (TE or TM) mid-infrared range radiation is demonstrated. Moreover, the tolerances of the designed structures' performance against the gratings' fabrication errors are tested, and fair manufacturing tolerance while still maintaining high peak efficiency along with a small deviation of its spectral position off initially predefined central-design wavelength is proved. In addition, the electromagnetic fields amplitudes and Poynting vector over the cavity-absorber area are visualized. As a result, it is inferred that the electromagnetic fields' confinement in the designed structure, which is a key to their upmost efficiency, is two-dimensional, combining in-depth vertical resonant-cavity-like confinement with the lateral microcavity like one set by the presence of gratings.