After seeing a demonstration of a straight ruler on a long arm, I ordered one (and a curved one because, why not?). It's 1/4 inch thick, and is designed to slide along side the hopping foot. I am really good at doing SID with my DSM, but I couldn't do free motion on it to save my life, whereas on the long arm, my free motion looks good, but I can't keep it steady enough to do SID to my satisfaction. I'm hoping the ruler will give me some stability. At least, it seemed to in the demonstration. She even showed us how to use a straight ruler to give stability when doing curves so it didn't look "jumpy", but I want to try baptist fans so I went ahead and ordered a curved one too.
Grannie Annie, my understanding is that a regular school ruler wouldn't work for a long arm because it's not thick enough and could slide under the hopping foot.