Cup holder. I'll shoulder in advance that it's probably a whimpy add-on, but I think I want one. Not for water. Not for beer (never for beer if riding). But for coffee. Sometimes I'm nearing a destination and would like to pick up some coffee on the way. But I can't b/c I can't carry it. So I think I want a cup holder, preferably one that I can put on and take off quickly so that I'd install it only when needed. Any suggestions?