Here I have fiberglassed over the depression. I started with neat resin, then leveled the center with thickened epoxy (colloidial silica), and then added concentric layers of 10 oz. cloth. I added some peel ply (release cloth) over the top to smooth it - that's the "windbreakery" looking stuff you can see. You can see the "real" drain over to the left in the photo.