I was thinking of drilling cable holes through the plywood and screwing the ceramic holder dircectly to it. Do the holders get hot enough to be a concern? The lights will be on for a maximum of 45 minutes. (I'm making a panel of 10 lights to have on when I am using UVB Philips tubes for Vitamin D.)
Your design is good but could be improved if you tapped a threaded hole in the aluminum that matched the screws. Then washers would not be necessary. I have been trying to fine a 1/8 ip threaded hickey to fit these. No luck yet.
