I don't think there's a property for ScatterSky that lets you render a bottom half to it. If you were to use the Skybox class instead, that does let you specify a cubemap and get a complete surrounding sky without the whitespace, but at the expense of the cool features scattersky has like the day/night cycle.
What you could
do is hide it, which will help in other ways too (nothing's more jarring than running into the edge of the world!)
The first is elevation. If you make the terrain itself higher on the Z Axis, the corners of the scattersky become a lot less apparent from the ground.
Also in this level there is a lot of terrain variation (default Outpost.mis). If there are hills blocking the whitespace, not only do you hide it but you also have a legit reason to prevent people from going to the edges and looking out into the abyss. Outpost is pretty big, only a small part of it is playable, the rest is just window dressing so it looks and feels like you're in the middle of something. Window dressing.
With intervening terrain, height, and designing the level in your favor, the last thing you can do to make it less obvious is adding fog to the level. I think even if you went to the edge of outpost you can barely see the black (or yellow I guess) just because it's fogged out and blends well.
There's also a waterPlane, which extends forever
and does a lot to obscure it. Not always practical though.
Honestly if you ask me your best bet is to design maps in a way that there's always something obscuring the blank space, like steep cliffs or mountains in the horizon, and raising your level enough that players can't see the edges of the umbrella from the highest point. Alternatively, Skybox is always an option if you want to sacrifice some options.