CFK sandwich board, strong and light, it can be made thinner i.e. with 8mm core. Foam core with 2x 300g CFK on each side should be enough. Rohacell would be a good core material but not cheap. AirexC70.75 works as well.
Rohacell: Rohacell / Hartschäume / Sandwich-Werkstoffe - Suter Kunststoffe AG
Airex: Airex C70.75 / Hartschäume / Sandwich-Werkstoffe - Suter Kunststoffe AG
If you want to build it cheaper, you can also use a wood core, just make sure the edges are coated with epoxy or paint to avoid water entering the core.
I built a sandwitch board with rohacell and a box for my inflatable with airex, see here: Daniels (un)geared inflatable (slowly built)
The plates are very strong and still in use.