An enclosure is filled with a filling gas according to the disclosed method. An enclosure is provided having an interior, a width, a height, a thickness, and fluid filling and exit holes fluidly communicating with the interior. Filling of the enclosure is commenced by directing a flow of the filling gas at a filling flow rate into the fluid filling hole. An oxygen concentration of gas exiting the fluid exit hole is sensed. The filling of the enclosure is stopped when the sensed oxygen concentration reaches a threshold concentration, wherein the threshold oxygen concentration and/or the filling flow rate are selected by a Decision Support Tool based upon the width, height, and/or the thickness.