The 12" Commercial Ektar has an unusual front thread, and no commonly available filters will fit. According to the 1953 Kodak Ektar Lens Catalog at cameraeccentric there's a special ring that adapts this lens for use with Series VIII filters. I'm not sure if the ring came with the lens or was a separately-purchased item, but none came with my 12" CE.

I know SKG can make one for me, or I could con a machinist friend into doing it, but finding a proper ring might be easier (or not, time will tell).