We report on the results of numerical investigations into the effects of slot geometry of horizontal slot microdisk resonators on their performance as optical biosensors. We find that for resonator-based biosensors, the mode overlap is the limiting factor for the performance. With this finding, we suggest that slot resonators are superior to conventional, evanescent-field based resonators primarily for the surface sensing, where a high optical field created by the thin slots can be used. Possible mixing of modes reduces the mode overlap and thus need to be avoided by proper design of the resonator structure. We further propose an optimized triple-slot structure to double the possible detector sensitivity.