The trouble as I see it is because you can't see the level in the reservoir there's no way of knowing if it's almost full or almost empty except by trying to fill it up. You can see the fluid in the filler neck when it's almost full but that's all.
My old Honda Civic also has a hidden reservoir but has a transparent plastic tube attached to the filler cap with a small hole in the cap. You put your finger over the hole and remove the cap and tube. The liquid is trapped in the tube and you can see the level.
On the
Ibiza there is a filter in the neck of the bottle and I don't yet know how easy it is to lift out. If it does come out then I might try a length of plastic tube if I can lay my hands on one.