Install securing against rolling on the sleeping floor for building acceptance

I can't see it clearly, but could 2-3 steel cables (like on the balcony) be stretched?

Otherwise: Milk/Plexiglass?

Barbed wire?

I would use a round timber, either like a thick curtain rod or pine trunks. There is also the gym rope variant.
Is there possibly something at Ikea that could be misused?
Copper pipe as an industrial style variant?

Thank you very much! I was already starting to become unimaginative with the many things that happen at the end of construction and am newly inspired. I will now go over, measure, and see what happens.

You can put any kind of railing you like there. There are so many stylish staircases... I wouldn't feel like cleaning glass.

You can also use a safety net (fall protection), for example in kindergartens. This can be ordered to size directly and is available in many colors (depending on how old the children are). The advantage over glass or plexiglass is that air can circulate better, which is beneficial under the roof.