Thank you Nathalie for your 2 great boxes. They were small and only single layer cardboard but because they were made of good quality cardboard and in great condition I thought it was worth trying something different and I am quite happy with the sturdy result.
For this one I used a different technique consisting in first sticking together many layers of cardboard before cutting to shape with a stanley knife or a jigsaw where possible to save time.
A bit of paint and here we are! I'll varnish it, put it on the wall and send the pics.