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.
Happy circles,
Frédéric