Rounding the corners of a slider

Hi, I have a Container COMPponent that contains 3 sliders next to one another. I’d like to round the corners of the overall rectangle made by the three sliders, i.e. not round the corners of each slider separately. I can round corners of buttons and fields as they have TOPs inside them that I can then modify. But how might I modify an aggregate created in a container? I was thinking if I could bring the Container into TOP land I can then create an overlay mask that could then be composited by the layering of Container COMPonents. Any suggestions?