Depending on how much of a recess you need, you can just cut a hole in the original firewall large enough to allow the mount to fit into, then laminate another piece of ply on the back. With a 1/4" ply firewall this would give you a 1/4" recess. The laminate on the back need not be as large as the original firewall. Continue the process and each layer will yield another 1/4". This method is heavier so use it on airplanes that will need nose weight anyway, like WWI biplanes!

Randy