With the Source engine, I want to make a map in the outdoors (similar to something like Dust2 or Inferno) but not let in leaks due to the open top part of the map. Is there a way I can do it without the leaks?
This is done with a skybox. In reality, the maps are still closed; they are simply using a special material which marks the space as to render the skybox, which is what ends up looking like the sky. Therefore, you must enclose the level with the "tools/toolsskybox" texture, while leaving sufficient space for things like grenade throws to happen. There are multiple versions of skyboxes, I suggest you read through the VDC article here.