I'm assuming you are talking about creating a round mask in photoshop with transparency and use it on the top layer of your composition, right?
You can also create a crop layer in [Advanced Output], select [Bezier] under [Point Mode] of the [Edit Points] in the output transformation and right click on the [Crop] and select [Make Circle]
Also, as an alternative to having a crop, you could use the same feature on any give slice but that'll compress your input selection into a circle and give the illusion of a half sphere. To do that, in [Advanced output], you'd select [Bezier] under [Point Mode] of the [Edit Points] in the output transformation and right click on the [Slice] and select [Make Circle]. Then you can scale it up or down holding the shift key in [Transform] to keep it round or turn it into an ellipse in either direction.
I'm attaching two screenshots to show the different results between the round slice and round crop.