A chap called Lee Frost used to use this camera lot and in his book "Panoramic Photography" he puts the screw mount lens adaptor on the lens then attaches the Lee Filter using blue tack.

Just looked at the book and he holds the filter in front of the camera lines up the filter then sticks it on the front of the adaptor. Blue Tack comes off easy as well.

Have done the same with my Hasselbald Xpan.