<p>Natural defects, particularly slit-like cracks, are a primary safety risk in critical metallic structures. While Eddy Current Testing (ECT) is a common inspection method, existing analytical models often fail to capture the complex geometries of real defects, such as wall thinning and roughness, by oversimplifying them as regular, smooth-edged shapes. This discrepancy limits the practical accuracy of defect characterization. To address this limitation, this study introduces an improved analytical model that incorporates a transition layer of intermediate conductivity to account for rough wall regions and wall thinning. Based on this, and in conjunction with the slit defect approximation methods proposed in prior work, approximate analytical models for cylindrical, elliptical cylindrical, and slit-like natural defects were successively established and verified via numerical models, with errors as low as 1.16%. Finally, experiments were conducted to validate the accuracy of four approximation for approximating slit-like natural defects as elliptical cylindrical defects, among which Approximation 4 (same length-to-width ratio and volume) achieved the highest accuracy, with an error of only 0.91%, with a 95% confidence interval (CI) of ±0.34%. The significance of this work is twofold: first, by considering the real-world issues of wall thinning and roughness in natural defects, this paper provides a more precise and realistic approximate analytical model for these defects; second, it provides a validated, high-accuracy analytical method for the practical characterization of slit-like defects.</p>