Consider an inline duct fan, such as one from The Sauna Shop's accessories area, for a soundproof solution. Additionally, you can install a duct silencer in your custom sauna kit between the fan and the ceiling vent for complete silence. The neighboring duct, silencer, and fan should all be strap-mounted and not in contact with any of the building's components. The optimum way to control this would be with a regulator, which would enable remote control and timer control of the speed.