Adding light below in an elegant way should not be so poorly executed. The frame is cast aluminium and the prism is acrylic. Mechanicaly the original deck prisms were destined to leak. They are destined for the scap heap.
The expedient solution was to remove the existing frame and clean up the opening. It was easy to cut flat piece of 1/4" clear acrylic and glue this in the existing opening using black polysulfide calking.
It's likely we will revisit this. While this solution has stopped the leaks it is not elegant. Glass prisms are available only they have slighly different measurements. The weather will need to be warm and dry to make the changes to the deck.
I did clean up and rebed the original acrylic prisms in the aluminium frames using silcon. While they might work they do not look great. The sun has crazed the acrylic and the ability to act as a prism is limited.
Here is an example of a more permanent solution.