What about a FMQ foot? You can sew straight lines with them, in addition to free motion work. They hop up and down so the quilt sandwich isn't pushed around.
http://www.april1930s.com/html/slant...y___darni.html This is mine, though you don't need slant needle. April and Jenny both have low shank ones. I bet your local LQS carries one. Mine does, but I don't like it as well as this one.