If you're wanting something that's not permanent you could use book shelves, then you get an added bonus of storage/shelving space! Just make sure to anchor them to a wall and together.
If they don't go all the way to the ceiling but you want them to you could use pieces of wood secured to the top of the shelves and the ceiling or you could use lightweight wooden boxes/crates secured to the ceiling and top of the shelf.
You could also repurpose pallets. A lot of the time you can find them really cheap or even free!
Hang a floor to ceiling curtain across the area you want to have the "bedroom".
A free standing screen, perhaps two put together would divide it off and easily moved if needed.
I lived in a large open space loft/apartment that my landlord creatively turned into rooms by using temporary walls. The had adjustable "feet" (similar to the screw down adjustable feet on your fridge) top and bottom. They were easily adjusted to form a tight fit between the floor and ceiling ........and they were easily removed and put in other configurations when I got bored with the look. It was like getting a whole new apartments without moving!
It was a few years ago now & im now in my own home so I can't ask him. 🤔 But I believe he just went to one of the big hardware stores......Home Depot , Rona, etc.
Possibly try an office supply place as they may have some moveable partitions as well.
( the ones I had, had a foam core inside, which is why they were so easily moved, if that helps when calling around to locate some.)
Good luck on your hunt sweetie.