I would assume you can make a diamond with the creating a region out of a rectangle in the trigger editor, but I need to add multiple shapes to a region and I don't see a way to do that in the the trigger editor.
I've tried doing this before also, but there is no way to get any polygonal shaped region other than a circle or a rectangle/square perpendicular to the x and y axis.
Hopefully Blizz will fix this problem by the time of release.
Haha, if I was gonna go through with that much trouble, I'd make hundreds of tiny 0.1-1.0 invisible unselectable invincible units and move them dynamically. I hope I won't have to do this. I've set this minigame aside until I decide to do such a thing or someone else does. =P