Since combustion requires oxygen, you can prevent the risk of combustion by placing the matchbox in an oxygen-free environment. This can be done either by removing all gasses from the environment, i.e. creating a vacuum, or by replacing the oxygen with another inert gas.
As the matchbox is fairly small, this can be done on a small scale with a sealable glass jar, a bit of wire mesh and a helium balloon. Place the matchbox in the glass jar and secure the wire mesh inside the jar (this just needs to be secure enough to prevent the matchbox falling out at the next stage). Turn the jar upside down, so the open mouth of the jar is the lowest point. Then release the helium from the balloon into the jar, since the gas is lighter than air it will rise and displace the air in the jar. When the balloon is empty, seal the jar. The matchbox will now be in a helium environment.
There's a good chance that the helium will slowly diffuse out of the jar over time but, if the seal is good enough, the larger air molecules won't be able to get in. So the effect will simply be to reduce the pressure inside the jar (which should increase the seal). Once the seal breaks down, it's easy enough to repeat the process with a new jar/lid.
An additional benefit, is that this should also prevent the risk of pest and moisture damage to the box.