My material of choice became fabric. After some searching, I settled on this product, in particular the “vlaggendoek” or “flag sheet” variety. This material weights just 115 grams for 1m2, which is conveniently almost the same size as an A0 (841mm × 1189mm). Printing + delivery costs just over 20 euros, which is actually cheaper than an A0 paper poster with a plastic coating. That’s not all: apparently the material is fire retardant, because you never know when fire could break out at a conference.
But the best thing of all? You can fold it and it still looks great when you unfold it!