This is iffy but do you remember the filter holder kodak made for the polycontrast filters under the enlarger? It has three rubber tipped setscrews that would mount to the lens and you slide the filters into the slot. With the proper orientation you might be able to slide an A series Cokin filter into it and use it sucessfully. I don't know if the cokin filters will need any modifications to fit, just that the size is close.