I have no expeience with clear duct tape, but my thinking is that it should work.
I built a canoe that held 3 people out of cardboard and duct tape. The boat never took on any water. The combined weight of the 3 passangers was ~550lbs. My experience is duct tape would work.
Fill the spaces with cardboard. Make sure cardboard has duct tape on both sides to prevent moisture. Put cardboard over area to be covered and fasten w/ lots of tape
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