Slightly off-axis digital holographic microscopy (SO-DHM) has recently emerged as a novel experimental arrangement for quantitative phase imaging (QPI). It offers improved capabilities in conventional on-axis and off-axis interferometric configurations. In this contribution, we report on a single-shot SO-DHM approach based on an add-on module adapted to the exit port of a regular microscope. The module employs a beamsplitter (BS) cube interferometer and includes, in addition, a Stokes lens (SL) for astigmatism compensation. Each recorded frame contains two fields of view (FOVs) of the sample, where each FOV is a hologram which is phase shifted by π rads with respect to the other. These two simultaneously recorded holograms are numerically processed, in order to retrieve complex amplitude distribution with enhanced quality. The tradeoff is done in the FOV which becomes penalized as a consequence of the simultaneous recording of the two holograms in a single snapshot. Experimental validation is presented for a wide variety of samples using a regular Olympus BX-60 upright microscope. The proposed approach provides an optimized use of the imaging system, in terms of the space-bandwidth product, in comparison with off-axis configuration; allows the analysis of fast-dynamic events, owing to its single-shot capability when compared with on-axis arrangement; and becomes easily implementable in conventional white-light microscopes for upgrading them into holographic microscopes for QPI.