That’s deceptively hard. In a void, it’d be pretty easy, but not if you want to have anything behind it.
The easiest way I can think of that works with complex shapes is to line up the UVs in such a way that the diamond occupies a square of space- preferably in a corner.
Alternatively, for this very specific use case, assuming you have the center of the object at the center, you can use a distance check of local position.
Change your mode from transparent to masked.