-Shane/SMBooth is 100% correct in stating that nowadays, just about all b+w film is sensitive to red light as well; my bad.
Originally Posted by chihiro
So - a toned piece of plastic -of any colour- (to minimize light entering in the first place) combined with a piece of tape or something covering the frame window when not needed (further reducing the risk of light leaks) will be your best bet. Also, taking care to have the film firmly placed against the back will make it even less likely that light entering through the frame window will cause any trouble.
Oh, and as an additional measure, try to keep the window out of direct sunlight when you advance the film to the next frame.