Does the Furuno 1st watch radar need a special mounting bracket or plate, or can I mount it directly

You can mount it directly to a metal plate under the canvas.  It should be level with the horizon while your boat is at rest.  If you want slightly better performance while you are on plane then you can tilt it down in front a couple of degrees.  You'll normally use your radar while the boat is going slow, so level with the horizon is generally the ticket.