Have you considered using an acrylic ruler? I'm not an experienced frame quilter, but I have seen rulers mentioned on the homequiltingsystems group at
http://groups.yahoo.com . For ruler work, I think longarmers like to use a foot that is extra thick. This foot is available for only some makes, but quilters also make their own by glueing layers of washers to the bottom of the foot.
Don't rely on my info, though! If you check out that group, be sure to read the files and do a search before posting a question; there's a lot of info there already.