How can I identify/select a foam for stiffening a bag (buying online)? I've started making my own bike luggage.  The first is a simple zipped frame bag, unlined.  Now that's finished successfully I want to make a full length toptube bag.  Because these sit on top of the bike they're usually stiffened with a sort of foam, about 2-3mm thick, to stop them flopping around.  The foam is trapped between the outer and a lining.  I need to buy some of this foam sheet, but the typical EVA foam sheets in craft shops (and kids' craft kits, so I have plenty in lurid colours) is too soft at about the right thickness.
I'm using waterproof fabric (ripstop polyester) and may seal the seams on this bag, but I need non-absorbent foam.  The zips won't be waterproof.
How can I identify a stiffer foam? Densities are given for the better craft foams, but how does that relate to stiffness.  I don't want something actually hard, as I'll only just be able to stand over the bag on the bike; it will be packed with soft stuff  on top.  As a bonus, the foam will provide a bit of insulation for keeping a drink cool.

I've added some links to commercial products for those unfamiliar with the terminology.  The main reason for making them is to maximise the amount I can carry without panniers (bike-packing style; cost is also a consideration.
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.