Dont do thisIf you have a leak, I would try to figure out where the leak is and fix it, or if you can't find it. Just put a skybox around your already made skybox. This will fix the leak because the leak would be contained within the second skybox. Now don't make the second skybox too big, that will just make the map size bigger and have to make the map render more. Just put the second skybox a square away from the original.
You can use the cordon tool for temporary sealing for quick compiles instead of creating skybox cubes around your level.
It essentially has the same effect but is clearly temporary, there's no physical brushes to get in the way and can be easily toggled on or off.
It looks like you have a skybox, I assume you have a light_environment? If you already do, try turning up the brightness. If that doesn't work or produce the desired effect, try scattering a few light entities around.