This is just a simple cylindrical bag that you could also use to carry a padded mat for camping or really anything that rolls up. You can add whatever designs you want. (You could easily do anyone from the Avengers like Iron Man, Captain America, Black Widow etc.)
About a yard of fabric (the exact amount will depend on what size bag you need, see step 1 for measuring)
Accent fabric or embroidery thread (depends on your design, I only got 1/8 a yard and was covered.)
Pattern making material or poster board
Step 1: Getting Measurements
My mat was 24" long and had a diameter of about 41/2".
To make sure the bag isn't too tight, add about an inch to an inch and a half to those measurements. I cut out a rectangle that was 251/2 inches long and 19" (6" x 3.14=18.84" circumference) wide.
Step 2: Making Patterns
Now you can cut out the patterns for your accent fabric. You can go nuts creatively with this. I just hand drew Hawkeye's mask and H. If you are embroidering your design, you can cut your design out in interfacing or embroidery backing instead of fabric.
Step 3: Applique and Drawstring
Next, cut a length of drawstring cord for the wide portion of your rectangle. Make sure it is longer than you need so that you can knot the ends (I probably used about 23 inches). The side of the fabric with the design should be facing down so that the seam for the drawstring will be on the inside. Fold the end of the fabric over and pin down.
Step 4: Finishing
Pin your circle to the bottom of the bag and sew. Turn the bag right side out.
For the strap, I cut a long rectangle that was 30 inches long and 4 inches wide (you can measure across your chest to see how long to make yours). Fold the rectangle's width in half and sew the length and one of the ends. Turn inside out, press down and sew to ends of the bag on the opposite side of your design.