Dimension for floor chipboard

[Lindgrenski]

I'm renovating the hallway and will eventually install a new staircase. But not until the basement is finished, which will take a few months as it involves casting among other things. I thought I would cover the old stair opening so we don't have to walk on planks to get into the living room. The opening is about 200x80cm. One beam (220x75) is exposed, but on the other side, I need to add a stud as the wall stands over the entire stud.
My idea is to add a 200x60mm and place flooring chipboard between it and the beam. I'll have to lay the chipboard across due to the width. Is it enough to add 45x70 or 45x95 between the beams to fasten the chipboard? 80cm is a bit much to leave unsupported, I suppose?

 

[anaitis]

Currently renovating the hall and will eventually install a new staircase. But not until the basement is finished, which will take a few months as it needs to be cast, among other things. I thought of closing off the old stairwell so we don't have to walk on planks to get into the living room. The opening is about 200x80cm. One joist (220x75) is exposed but on the other side, I have to attach a stud as the wall stands over the entire stud.
My idea is to attach a 200x60mm and to lay flooring chipboard between it and the joist. I will have to place the chipboard crosswise due to the width. Is it enough to attach 45x70 or 45x95 between the joists to secure the chipboard? 80cm is a bit much to hang freely, I suppose?

Both of your suggestions work well,