I was not sure about what to use, so I took the largest off-road sponge that I could find from my dealer, because I didn't want to obstruct too much the airflow.
I'm pretty confident that is ok, because sometimes I hear the fan running, but after some minutes it stops, so I guess the fan is still able to cool down the battery.
At first I thought about using paper as Toyota is doing nowadays, but I couldn't find the "right" one for filters.
After having purchased the foam, I thought about recycling a face mask, but it was too late...
Funny thing about the foam, it's quite had to exhale through it, but I can inhale almost as normal.