Buck-The problem you're having is the exact reason for mounting spot lights on the front of the bow pulpit instead of the cabin top.
Why not make a round flat metal "shield" of about a foot diameter with a hole in the middle for the shaft and mount it right under the white "steaming" light? I don't think it would be illegal as long as the light can be seen by other boats, and it would stop the light from shinning down on your deck and bow pulpit and blinding you. Just a passing thouht, but I think it would work, perhaps with some refinement to the shape and/or size of the shield. Make one out of cardboard and experiment with it. Cheaper than stainless to work out the exact shape details on! Joe.