Giant paper popup.What Strong material to use other than Carboard?

What material could you use to make strong seats out of this?

crushbabe1 month ago

You could try a German product call WellBoard

Sign makers use plastic "waver board", no clue how the stuff is called.
Basically looks like cardboard but instead of the wave pattern of the inner layer you have vertical strips connecting both sheets.
The stuff is also available in good hardware stores but only in sizes of up to 2x1m in my area.
Available in different thicknesses too and quite still and durable.