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.