Fluorescence spectroscopy is commonly employed to study the excited-state photophysics of organic molecules. Planar Fabry-Pérot microcavities play an essential role in such studies and a strategic cavity design is necessary to attain an enhanced light-matter interaction. In this work, we computationally study different geometries for a planar metallic Fabry-Pérot microcavity tuned for the absorption of Sulforhodamine 101, a typical dye for fluorescence spectroscopy. The cavity consists of a polymer layer enclosed between two silver mirrors, where the thicknesses of all the three layers are varied to optimize the cavity. Our transfer-matrix and finite-difference time-domain simulations suggest that a cavity with 30 nm thin top mirror and 200 nm fully reflective thick bottom mirror, thus having only reflection and absorption and no transmission, is an optimal design for maximizing the Purcell factor and spectral overlap between the cavity and molecule, while still sustaining an efficient measurability of the fluorescence.