There’s currently a sell-off of a 100 Liter (no idea much this is cubic inches) inflation bag on German eBay and similar sites, for around 5 Eurones. Just in time, as I anyway wanted to build an XXL inflation bag. The Produkt is called Fluxbag: https://www.google.com/search?q=fluxbag&rlz=1C1CHBF_deDE790DE790&source=lnms&tbm=isch&sa=X&ved=2ahUKEwiAztLe-ezpAhU-QUEAHUhRADIQ_AUoA3oECB8QBQ&biw=1920&bih=888
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