Believe it or not, I made the mirror cover using two sheets of 1/8” low grade packing plywood from the bottom of a shipping box. I epoxied the two pieces 1/8” plywood together, cut out a circle of 27 ½” with my router, and rounded over the edges with a 1/8” rounder bit, and coated it with several coats of epoxy. I sanded everything down to 320 grit, and painted it flat black. Looks great. Couldn’t have come out better. The mirror cover is secured with three 1 ½” circle cutouts from BB ply that are epoxied onto the baffle, and lined with felt. I actually made these from felt lined cutouts that Steve used to stack mirrors.
Pictures below show the process: