How about taking a small black plastic bag, cutting the bottom out, making it a tube, and then slipping it over the camera bellows, and attaching it (larger rubber bands?) to the camera body on one end and the lens standard on the other. The bellows will block the bulk of the light, the plastic bag on top of the bellows will prevent the light reaching any of the leaking areas.
Not a permanent solution, just something to use until you find a replacement bellows.
You know something? That's not a totally shabby idea.