of course paper-based. For the outside, I used
Decopatch glue, as it glues and varnishes at the same time, making the cardboard even stiffer.
For the inside, I used wallpaper glue, which costs less than
Decopatch. Disadvantages: much longer drying time, the substrate drinks the glue, the paper doesn't stretch like with Decopatch and it's full of wrinkles...