I have a long 24" clear ruler that's 5" wide and has a lip that hooks at the edge of my cutting mat so i know i'm always keeping my cuts straight as long as I've lined up the fabric on the selvage. I was looking for one for a friend who's new to quilting and I couldn't find another.
That said, I'm not the worlds most precise quilter as I think this is something that's my stress reliever, it's not supposed to add stress, so I don't freak if something isn't perfect. These are homeade, works of art, they're not supposed to look like a machine made them. (just my opinion)