How can I identify/select a foam for stiffening a bag (buying online)?
What you're looking for is foam interfacing (Pellon Flex Foam, Annie's Soft &amp; Stable, Bosal In-r-Form, etc). This is a soft foam material generally about 1/4&quot; (6mm) thick, designed to be sewn into projects like bags, totes, and hats to provide them with a stable structure while still allowing them to flex--it's meant for exactly your intended use. You may feel like it's &quot;too soft&quot; when handled, but when sewn into the shape of your bag, it will hold the form very well (especially once it's filled with other items). As far as your waterproofing concerns, the stitching across the seams should be sufficient to prevent it from getting wet in any weather you'd want to be out in yourself.
The blog Seasoned Homemaker has some information about the three main brands, as well as how to work with this type of interfacing and examples of projects made using foam interfacing for their structure. I've personally used it to make a witch hat (the pattern designer's work is shown here, also using foam interfacing for the structure) and have found it to keep its shape extremely well, even in the flat shape of the hat's brim.