Rick beat me too it . Except as I look again, I realize that you said that yours is a D2, not a D5.
So I think you need to go to the "Lens Mount" section on here instead: http://www.khbphotografix.com/omega/.../D2.htm#Mounts
Based on that, I think you need a 4 1/2" cone, and either a 42mm, 52mm or 62mm Flat Lens Disc (where the size corresponds with the size of the hole).
The link that Rick posted makes it seem most likely that the lens has a 53mm thread.