The Scottish Tartans Museum sells clan badges made in sterling silver. I've heard of police/fire personnel wearing their badge, especially if they're retired, on the bonnet. A little sprig of your clan plant tucked into the ribbon can look very nice in lieu of a badge. You might find a brooch at an antique store, or maybe your grandmother passed hers down through the family and you inherited it. There is no rule about what you can wear on your bonnet, unless of course you're subject to military/pipe band regulations.