Hello. I have a Lennox 80UHG furnace, without a filter. It has an electronic/static type of filter, but it doesn't work anymore. I want to convert to a normal 20x20 slide in filter. Any idea on the cost for an HVAC guy to modify the ductwork to accomodate the slide-in filter? Just a ballpark is all I am looking for, so assume a "typical" installation. Thanks.