A model for the dynamics of non-polar polymers close to the glass transition, based on percolation of slow subunits, has been proposed and developed over the past ten years. By extending this model, we have developed a 3D model, solved by numerical simulations, in order to describe and calculate the mechanical properties of polymers in the linear regime of deformation, with a spatial resolution corresponding to the subunit size, around 3 nm. We calculate the evolution of the elastic modulus as a function of temperature, for different film thicknesses and polymer-substrate interactions. In particular, we show that for strong interactions and small thickness, glass transition is not only increased (by up to 20 K) but also broadened and accompanied by huge fluctuations of the storage modulus. This effect due to the low percolation threshold across thin films conjugated to the huge difference between high and low frequency moduli, should be more apparent in mechanical experiments than in dielectric spectroscopy or calorimetry.