Here is a quick and cheap design for a flash diffuser to fit the popup flash on the Nikon D200.
Cut the following template out from a (clean) one-gallon plastic milk bottle using a sharp knife or scissors:
Pay attention to which part of the bottle you use. I suggest the 18 by 20mm tab be taken from near the top/neck of the bottle. Then the contour of the bottle will hopefully match nicely the contour of the rear of the popup flash assembly. Also the 18 by 20mm tab will probably be made of thicker plastic and this helps make for a nice rigid fit into the hotshoe.
Bend the plastic at two points; the 18 by 20mm tab should slide directly into the D200 hotshoe. The 40 by 50mm section should hang down over the front of the popup flash: