Wearable self-injection-locked (SIL) radar requires near-field sensors that preserve injection coupling at a millimeter-scale gap under bending and placement variations. This letter introduces plane-of-interest metrics, including uniform-field coverage (UFC), relative standard deviation (RSD), and bending robustness factor ρA , to guide robustness-oriented design of ultra-thin flexible near-field sensor for 5.8-GHz SIL wrist sensing, and demonstrates the workflow with a slot-type implementation on polyimide. Full-wave results show that slot-type implementation maintains a strong and spatially uniform field distribution on the plane of interest under wrist-like curvature. System measurements across multiple bending radii and alignment offsets on the same SIL radar platform confirm a robustness trend consistent with the ρA ranking. https://ieeexplore.ieee.org/document/11459171 DOI: 10.1109/LMWT.2026.3676890