I would definitely build these as a corrugated construction with vertical corrugations. Add to that some reinforcements going across that are shaped so they bond to the fulle profile of the corrugations, and you will have a very strong and light construction. You could use a standard plastic or metal corrugated roof plate as a mold for making the panels.
The corrugation wil provide two advantages the first and obvious being the much greater longitudinal strength of the "elements"
The second advantage is a bit more subtle, but logical, the waves that may hit the panels will have some of their impact energy dissipated when hitting the corrugated surface.
I would build them of a combination of kevlar and carbon fibre, to make them very impact resistant, combined with a core of maybe up to 5 mm thickness.
I do not think you would need high grade Carbon fibre, the EC "black Stuff" combined with an outer layer of Kevlar /carbon mix twill and maybe something like the discounted biaxial CF .
Interesting application of "advanced composites"

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