To describe the situation a bit more:
I have an outdoor window with a metal frame. There is about 10-15cm gap along the bottom of it. Tiled floor. The gap is not even height, so it's literraly about 10-11cm on one side and it widens to about 15-16 cm towards the other side. The floor is tile.
I need to fill this gap with something that will prevent water/dust flowing from one side to the other (not to worry: each side has a drain) and I was thinking to use some kind of rubber and cut it to size. But given it's somewhat un-even height I thought there might be a better material? Something easy to work with. Maybe something originially soft and it hardens when exposed to air and maintains waterproof quality long term?