The first attempt was made in a hurry and we soon realized that it was not strong enough to lean on. Also the foam was too thin – the cardboard back unstable and I had 2nd thoughts on the used fabric. So it had to go. The new plan is single square pieces with thick padded foam on 0,5 mm waterproof multiplex (plywood) which is glued to a sturdy piece of plywood. The new fabric is half shiny and strengthened with a layer foam glued on the back. (looks a bit like the curtains).
We still want to be able to use the window so a part of the board can be removed. The other half is permanent installed including a little cabinet for water and phone charger.
On YouTube they make it sound easy but it is not. If you do not own an air tacker it is hard to have the same pressure. End result pull stripes on the side. Aaarch.. that is not what I want.
The single parts are mounted to a plywood piece and you can still see the ugly sides but this is it for me (at least for now).
We also finished the wall curved valences so we have a bit more space to sleep.