help needed for propagator

hello all we are thinking of building a heated propagator to go inside the potting shed it needs to be about 4ft by 2.5 ft still working in feet and inches showing me age there lol, ive been looking at some soil warming cables plus thermostat .now what length of cable would i need and would it need a metal bottom or is ply ok and has anyone got any plans or ideas thanks for looking phil/anne