If it's really just constant transparency (not a mask or gradient or anything), a pair of WinAPI functions can be utilized. I wrote a mini-post on the matter:
https://yal.cc/unity3d-window-alpha/
Viewing post in Screenshot effect question.
If you create your polygon shapes in advance (e.g. using window_shape_create_polygon_from_path) and use window_shape_set_nc, the underlying Windows functionality is pretty performant - in my test with flipping between two different dinosaur-based shapes the operation takes between 0.5-1ms on average, which is more than enough to do this at 60fps.