expanding on the answer from @ZackBeNimble
It might be easier to use degrafa:
skin component:
<degrafa:GraphicBorderSkin xmlns:degrafa="namespaceLovin">
<degrafa:geometry>
<degrafa:GeometryComposition>
<degrafa:RoundedRectangleComplex topLeftRadius="0" topRightRadius="0" bottomLeftRadius="5" bottomRightRadius="5"
height="{skinHeight}"
width="{skinWidth}"
fill="{someFill}" />
</degrafa:geometry>
</degrafa:GraphicBorderSkin>
you'll need a fill. also, skinWidth and height are degrafa internal vars that they added in the beta -- you'll have to override updatedisplaylist if you're using an older version to get the unscaledwidth and unscaledheight.
then in the component you're skinning, set this style: borderSkin="package.class.location"
Might be a bit quicker if you're not familliar with traditional programmatic skinning... i'm more a fan of el degrafa.