Just checked and there's now way an M42 screw hole will fit. Should have looked before posting. But something like this could work, possibly would work with the n900 as it is.

Best left unsolved for the time being. I'll call into the camera shop in town and see if they have any ideas but I doubt they'll be anything half decent that will fit. It'll need to be 20mm to stand a chance of being directly over the camera.

http://www.ebay.ie/itm/10X-Optical-Z...item2c650e9032