Where will you be camping? If you're in any sort of established campground, any self-made case isn't going to be sufficient.
The problem is more legal than practical. There is an established process for approving bear-resistant devices, and anything that hasn't been through the process and been certified is not considered legal. If a bear gets into it, you could be found legally culpable. This can range from a citation if a ranger is annoyed that you had an unapproved container, to a serious fine and/or criminal proceedings if the bear has to be put down or relocated, or if the bear is injured by trying to open your stuff.
If you are not in an established campground, where you are less likely to come under the scrutiny of a ranger, etc., you may get away with it. Anything inside will likely be tossed/destroyed, since the bear's first maneuver is usually to knock things around and jump on them for a while, before trying to hook a claw into the gaps.