It kind of depends on how stiff the cowl is. When the nose of the plane is to be covered in monokote I usually use 1/16 plywood glued to the spinner backplate as a spacer. For a fiberglass cowl I use 1/32 or1/64 ply. If the cowl can flex you need a little more space. I glue the ply on with rubber cement so I can easily remove it from the backplate.