If you want your pictures to be free from lightleaks, it's very important that no light at all can get into the camera other than through the pinhole! Black electrical PVC tape is very effective at keeping out light.
The most important places to seal are between the film canisters and the matchbox. Place strips of tape down the front on both sides. Use two layers and make sure it is stuck down firmly all around
Pay attention to the ends of each reel. Add more tape here, trim around the spindles so that the tape adhesive doesnt stick and prevent the film being wound on. Again, use a couple of layers and check all around both joints to make sure they are totally sealed.
The cardboard of the matchbox will also leak a small amount of light, especially in bright conditions. This will give your photos a mottled red effect. If you don't want this, tape all over the back and sides of the box so that no cardboard is showing