There are two ways to do this. Both start by doing: **New > PCB Lib**  **1) Less accurate but easy to do.** Start by building an **Multi-Layer**, **Oval** pad with a **Slot** hole but with an annulus that is within the width of the flat section:  Then overlay a **Solid Region** and then drag the green circles to move the vertices, drag the blue circles to add and drag more vertices. Double click the green vertices to remove them:  **2) More accurate but more fiddly:** Select **Multi-Layer** and **Polygon** then edit the shape of the pad using the **Edit Points** dialogue:  Then edit the **Slot** hole shape and position. * In either method then add the vias for the extra conductivity to other layers.

Thanks, I will give the 2nd method a try

So I chose method 2 for this, but i used an external cad tool to extract the required x,y coordinates of the outline first. I found that for a quarter of a circle, 4 points gives enough resolution to make it look enough like an arc. For the side with the semi-circle, I used 9 points.