Choice of flooring, storage above toilet shower

[lala_Se]

I have a small storage space, 2.4 m by 0.9 m above the toilet shower. 0.5 meters high. I had a board that ants thrived in, now I've removed the board. What should I put in instead, plastic strips? Varnished wood strips? Metal? Probably good if the area is ventilated.

 

[BirgitS]

If it's humid, maybe a våtrumsskiva is a good choice?
Or if it's dry, so that the only requirement is that it isn't organic, a golvgipsskiva?