Four efficient algorithms that have been recently developed provide a direct solution of certain canonical singular integral/integrodifferential equations formulating electromagnetic scattering by arbitrarily wide strips or slots either in free space or in the presence of separating boundaries. In this paper, two of these algorithms are used for investigating the scattering properties of a slot on an infinite ground plane loaded a) by a cavity with a perfectly conducting semi-cylindrical wall, or, b) by a dielectric semi-cylinder surrounded by air. Plotted results, which are presented either for E- or H-type plane wave/line-source excitations, bring to light the effect of varying several geometrical parameters on the response of the structures. The characteristic current problems for these same configurations are also investigated.