You should be able to do this:
TSharedRef<SBox> MyBox = SNew(SBox);
TSharedPtr<SScissorRectBox> ScissorRectBox = SNew(SScissorRectBox)
.Content()
MyBox
];
However to me seems like that “SScissorRectBox” class is incomplete.
The Construct() function should deal with this stuff by itself since “FScissorRectSlot” constructor won’t get called by SScissorRectBox’s default.