You can get all of the Westalee rulers for a longarm machine. They will work great on your sit down HQ 16, and they don't cost more for the thicker rulers.
Right now, HandiQuilter has a ruler of the month program going on. This month's ruler is the "Line Grid Ruler." It has lots of lines printed on it, and you can work inside the ruler, or outside the ruler. It's a pretty awesome straight edge ruler!