Floor in the attic - material recommendations?

Hello,
we have a so-called cold room above the ceiling of the upper floor in our house, which is only 1.30m high at the ridge and is intended only as a large storage room (approx. 120sqm).
The insulation (220mm) is directly on the ceiling above the upper floor and only today did we notice that the floor is missing. Walking and pushing boxes on the insulation is certainly not good.

What is recommended for this? It should of course be as inexpensive as possible, we had not budgeted for the costs and it will be rarely used anyway.
Thanks in advance.

We used tongue and groove boards. It was recommended by the general contractor, supposedly better than OSB boards. But we didn't cover everything with it, we don't need that much surface area. I got it pretty cheaply from a sawmill, it doesn't have to be the best quality for that.

Be careful because of the moisture. Friends covered the floor with OSB in the new building and then the insulation started to mold. No idea if the moisture from the living spaces rose upwards or what went wrong there.

We have also discussed the topic with our construction company. One could install an open battens system over the insulation. We should definitely not lay OSB because of the risk of mold.

The air must be able to circulate between the floor and the insulation. If necessary, double up with battens and achieve the required distance.

The air must be able to circulate between the floor and the insulation. If necessary, double up with battens and achieve the required distance.

We had 6 cm doubled up in the walkable area of the attic, and ESB boards (ESB is not a typo by me) were laid on top. Just try googling it.