Prayer Shawls can be rectangle or triangle, doesn't really matter. They are usually longer than a scarf so it can wrap completely around the front so feel more like a hug.
As far as an actual pattern to follow, sky is the limit. According to "legend" the knitted or crocheted shawl was to have a prayer attached to each stitch for the recipient. You can do the same with a quilt but the prayer would be on each piece of fabric on the quilt.
Hope this helps. Basically if you are making it a quilt you could think of it as a very long table runner.