Here's the tool that I used when I longarmed my Lucky Stars quilt. I bought it at a garage sale for $2. It's a Martha Stewart craft item. The instructions included with it shows how to cut even strips into paper to make paper lanterns.
It helped me evenly space the straight-line quilting without the use of a ruler. To keep the lines straight I just locked my horizontal lock on the longarm.
Here it is on the frame: