Have several "small" projects I've been trying to get to. One of them is to install some speaker in my rear seat side panels. Since the ashtrays are a legacy item for me anyway, so I sized up some Pyle speakers that are compatible with my audio system setup and found some grills on Ebay. Had some 3/8" birch plywood laying around from my trunk project, so got my Roto-Zip and off to work I went. The metal piece is something I sourced at the local Lowes hardware store in the electrical section. The diameter is almost exactly the same as the speaker....
I also use "elevator bolts" since the tops of those are very thin. The whole plastic piece will be covered in thin insulation and vinyl anyway..