Even a standard mast gets water down the inside when it rains. Consider it a free bilge rinse. You should have an automatic bilge pump switched on to handle it. It is amazing how much water comes down inside my mast in heavy rain from just a few holes where halyards exit.

Don't try to block the water flow. Even a few drops of salt water, sitting at the point where you try to divert the water will cause rapid corrosion. I saw a mast so badly corroded at an internal baffle that it almost split is two.