In this paper we show how a proper dimensioning of a magnetron cavity and its inner bulb allows one to design a hydrogen maser atomic clock with an arbitrary low frequency-temperature coefficient (FTC) for its cavity. The control of this parameter is of primary importance for the overall stability of the atomic clock. To this aim, a model based on finite element method (FEM) has been developed to simulate thermal expansion of hydrogen maser cavities and evaluate the FTC. The model has been confronted with direct experimental data and very good agreement is observed. A key geometrical parameter is identified that allows for a strong tuning of the FTC, so as to set it down to values \textit{a priori} as close as desired to 0, with limitations only dictated by the machining tolerances of the mechanical elements of the hydrogen maser cavity.