I'm not sure I can explain how I would improvise a pattern, but I'll try, Tape together a few sheets of newspaper so you have lots bigger than you think you'll need. Do it straight, since this will be the ouside edge. Fold both the paper and quilt section in half, with straight edge outside, and folded edges matching. Draw line to match. that will be your sewing line. take fabric away, draw another line 1/4 out from the first, and cut.unfold and check for fit. There are probably formulas to determine it, but a lot of my quilting is improvised.