It is shown that a bare simple fluid model (SFM) proposed some years ago for studying adsorption between two semi-infinite solid walls can be improved by modifying the surface term in the grand potential for the film phase. Such a correction substantially improves the agreement between the predictions for phase transitions provided by that SFM and results obtained from calculations carried out for ⁴He with the density-functional method at zero temperature. The corrective term depends on the strength of the adsorption potential and observables of bulk helium.