After carefully removing the brackets, we used one of them as a template, first tracing its profile onto a scrap of plywood, then cutting it out with a jigsaw and sanding it smooth.
The original brackets were about 1 1/2 inches thick, but I wanted the replacements to be a little thicker, so I added a piece of 3/8-inch PVC in between, then glued the three layers together with Trim Tight PVC glue.
We finished up the edges with light sanding. We filled the screw holes with Bond&Fill, a filler designed to work with all the available brands of PVC trim.
We predrilled holes for attaching to the house, then glued and screwed the brackets to the corner pieces.