Actually, the plastic lid is the most likely culprit (IMO). Even with black paint inside and out, that lid will still be prone to leak light at the edges, where the paint doesn't cover well. For my time and effort, I plan to stick with steel cookie tins for my camera making; I've got a Starbuck's cofee tin (originally held four or five one-pound bags of coffee) that will become a 4x5 stereo camera. On all of these, I only need to blacken the interior, install film holders, pinholes, and shutters, and possibly a plug insert in the lid to block single reflections coming around the tin's seam under the lid. Never a question if the steel is lightproof.
If you're more interested in results than appearance, or don't have access to folks who eat of lot of cookies around Christmas time, you should consider quart and gallon paint cans (the kind with the very tight lids). Very simple to work with, hold very large negatives, and the lid *is* light tight, without question, as long as it's not bent.