There's not much you can do about the empty spaces, Rick. Unity determines that you terrain must be square and that ist that.
If you want to get cute with it, I suppose you could rotate the golf course by 45 degrees so that it occupies the diagonal of the square - that would enable you to reduce the size of it a little and end up with less empty ground.
(You would have to be careful, though, since the height map produced for you by Jimbobh would need to be rotated and resized exactly the same way.)
Couple of not at all vital suggestion for your PS layout. Below is one of mine, as usually fictional: To what you have I added an overlay containing a distance grid made to scale, giving me a better feeling for distances. Each little square is 10y. And I also drew in play lines, from tee to shot point to the center of the green, because once you're in there, close-by in Unity, these will help show elevation changes and inform you about the most likely axes of play.
As I said, not at all crucial, just idiosyncratic.