There are a few ways to accomplish the top flange (the rim or one-piece euro-brace) of a sump. Originally, I would build the sump, squeeze in pieces of wood inside the upper edge of the sump, and use a router with a ball bearing guide to trace the lumber. This was tedious, and cutting wood for each sump would end up costing too much.
Later I bought an Edge-Guide that mounts to the router. Once the sump was built, I could use the edge guide to follow the outer periphery of the sump while the bit cut out the opening. This is fine, but it is possible to make an error, or even damage the sump if the center piece drops inwards too hard. Plus the sump would fill up with shavings that had to be removed afterwards. Some images of this procedure are included on this page when you scroll down quite a bit.