I don't think you will find anything.the difference is half a millimeter and that would require some might fine machining and metal thin enough and strong enough.You might try epoxying a 39mm ring to a 40.5 to 40.5 adapter ring by jamming it in. It willl be a tight fit I would think.
Most all the Horseman 6x9 lenses are 40.5 filter size. If you can't find an adapter through the usual channels try searching for the Horseman step-up ring part number 25682.