It's a little roomier, so you can prob also throw a little thing of wipes in there too! It zips and has a loop to fit around your wrist or hand from the door when the seat is in use. With the curtain lining for lining, the bag is also great for an ick bag in a diaperbag!
Start with a zipper at least 14 inches long, 2 inch strip for the handle, two 11.5 by 11.5 squares each of lining and outer fabric (I use curtain lining for the inside, like in an ick bag). Keep 2 scraps at least 1 inch by 1 inch handy for the zipper
fold the 2 inch strip in half, open, fold the edges in so you have a .5 inch strip... sew end closed
Cut out 1.5 inch squares at the bottom of each piece for your box corners.
Get your scrap pieces ready, sew one towards zipper from bottom. Then flip and press, and top stitch
repeat with other side, so it looks like this, the tabs should be 10.5 inches apart
take a main fabric and lining, sandwich your zipper and sew.
make sure right sides are together, and main fabric is facing the zipperpull.
fold back and press, then carefully topstitch. Repeat with other side.
attach the handle to one side, where the zipper closes, about 2 inches down from zipper. stitch in place with a 1/8 seam.
with zipper halfway open for turning, fold so main fabrics are right sides together and lining is together. Stitch the sides and bottom, leaving an opening in the bottom of the lining to turn.
sew your box corners, turn, close the hole in the lining, and you're done!