9 1/2" x 19" piece of Coroplast
or other corrugated plastic sheeting - most often used for making outdoor signage.
Some bungee cording (about 54", give or take).
My local Michaels store has 21 ft. bundle bungee cording on clearance, for about 74¢ plus tax per bundle.
A tool to poke some holes with
Using a 1/4" hollow punch and a hammer or mallet would be my recommendation to make nice, clean, round holes.
Optional: a #0 grommet setting kit would give your cords some added protection from wear.
or Xacto/craft knife, to cut other stuff with.
To print-out of this template (2 sheets)
*note: If you cut out and affix the shapes to the coroplast, there should be an approximately 3 1/2" gap in the middle between them.
A reusable (woven) plastic shopping bag or tarp
I thought this large plaid one with the zipper would work well for this, and they are easily found in either flea markets, or else those weird, quasi- flea market shops such as 99 Cent City.
double-sided weather strip tape
is the easiest method i've found for adhering woven plastic to itself. It forms a pretty solid bond, that is often stronger than whatever the first layer of tarp or plastic is.
Needle and thread
It is possible to hand-sew the entire bag together with a needle and thread, instead of using the weather-strip tape.
Otherwise, you'll find these useful to create a new zipper stop at the length of the bag, should the zipper of your bag need to be shortened.