Nanoporous membranes are widely used in (bio)sensors, chromatography, charge storage devices, filtration and catalysis. With increasing efforts to develop microfabricated sytem- or lab-on-a-chip devices that perform a wide range of functions, there is a need to devise in situ methods to incorporate elements such as membranes. A suitable in-situ fabrication method is electropolymerisation of monomer units around templates that are subsequently eluted. However, whilst this strategy can be depicted in a cartoon, direct morphological characterisation of any subsurface cavities is notoriously difficult. Often desirable pores are ~20-200 nm and thus on the length scale accessible with offspecular reflectivity on OFFSPEC. Therefore here we seek to use this technique to probe the number density and size of cavities generated by an elution strategy, thereby adding quantification to the cartoon.