It's very easy. I cut out a circle of plywood 3mm thick, the diameter of the mirror can. Then in order to get it to fit in between the trusses, I cut it in half and cut out those little squares where the trusses were, and then glued a lip on one of the straight edges to make a complete dust seal. I match marked the trusses and the plywood , gave it a coat of varnish and hey presto. To fit each half on, you can bend the ply a bit to make it fit. Very nice
