Hi,
Bit of a late reply…
But since this is one of the first hits in google concerning rounded UMG elements in Unreal, it might help others.
To mask multiple UMG elements you can use a retainer box, which uses an “effectMaterial” to apply some postprocessing to your widget. Use a UI material with translucency and use a mask texture or some math to create your rounded corners.
Anyway, not going to type an entire tutorial, but google RetainerBox Unreal and you should be able to work it out.
Grtz