In the end I designed my own bag based on Leesa Chandler's Pinwheel base bag, as demonstrated on Quilters Companion Issue 52 DVD (Handbag Workshop with Leesa Chandler). She also demonstrates how to draft the pattern for this bag (to whatever size is desired).
I used this pattern and these dimensions for my bag: Draw a rectangle 7 3/4 inches (20 cms) wide and 16 inches (41 cms) long (I extended the sides on the one in the photo below). Then add a triangle to the bottom, with sides 5 1/2 inches (14 cms) long and making 45 degree angles where it meets the rectangle (and a 90 degree angle at the base).
Step one
Cut out 4 sides, making one an extra 1-2 inches (2.5 - 5 cms) longer on the side. I used two fat quarters of complementary fabrics.
Then cut the longer piece into 2 pieces, 6 1/2 inches (17 cms) from the top.
Attach the wadding to the outer bag pieces.
Sew narrow hems on the cut edges of the extra long sides of both the outer bag and lining pieces.
Step two
Sew the base of the bag: Right sides together, join two contrasting pieces together along the bottom slope; repeat with other two pieces. Then sew a seam right across the two joined pieces, making sure that they are aligned to alternate fabrics.
Step three
Join the sides of the bag together: Pin the side with the hemmed split so that the two finished edges are against each other, pinning all this to the adjoining side.
Then work your way around the bag, sewing adjoining sides together.
Make the lining the same way, but leave a gap near the base of one of the side seams (for turning later).
Step four
Make a cord sleeve: measure around the top of the bag (mine was 28 inches), halve this amount (mine was 14 inches) and add 1 inch to this measurement (mine became 15 inches). Cut two pieces of one of the outer bag fabrics, each measuring 2 inches (5 cms) wide and your final length long (i.e. 15 inches for me). Fold the ends of each length of fabric over 1/2 inch and iron down, then fold each strip in half and iron.
Step five
Joining everything together: insert outer bag into the lining, right sides together (the outer bag will be right side out and the lining inside out). Then line up the seams of the outer bag and the lining, and insert the cord sleeves between the outer bag and the lining, starting in the middle of the fabric that does not have the gap in it. The ends of each sleeve will meet in the middles of this fabric. Sew all the way around the top of the bag.
Turn the bag out through the gap in the lining, and then sew the gap closed.
Step six
Threading the cord through the cord sleeve: Cut two pieces of cord long enough to go all the way around the top of the bag, with extra for tying together into a knot. Thread each through the cord sleeve all the way around the top (I attach the cord to a safety pin for this job), with each cord starting on an alternate side of the bag. Tie the ends together in a knot when the cord is through. Now it is easy to pull the cord up if you pull from each side simultaneously.
Step seven
Finishing the gap: Turn the bag inside out and pin the lining to the bag at the gap and then hand-sew the two edges together on both sides of the gap.