Unreal Engine Rounded UI Material

This approach allows the roundness to be easily controlled by one parameter, automatically rounding and scaling the Box Dimensions so that the box is always perfectly in frame.

Just make sure to set the Material mode and Blend Mode to 'User Interface' and 'Translucent' respectively in the Material Details.

A large rounding Material Instance

A small rounding Material Instance, notice how the Roundness parameter automatically handles the scaling of the box so that it stays in frame