Here is my design for a cardboard stool, made from 22 interlocking trapezoidal hollow forms, and capped with 3 layers of cardboard to form a seat.
I used 3mm corrugated cardboard to make the forms and then glued them together with pva.
The stool is sturdy enough for me to comfortably stand on (and i weigh around 80kg). In this design the central load bearing axis is comprised of 66 layers of 3mm board - pretty sturdy. The photos show its condition after about 5 months of use, which explains the slight wear.
I've included some schematics so you can get a better idea of how the piece works, if people are interested i'll consider adding a step by step guide.
You can check out some of my other cardboard pieces here