Deck material

Hi fellow Sailing Maniacs
Now for this winter's project:
I have a 27' GRP fishing boat hull - originally with a deck below (2-1½')
the railing.
In order to make more use of the room inside, I would like to make a new
deck on top of the railing.
The railing is about 3" wide very solid, inverted U shape (flat on top)
So we have a good flat surface to connect to.
Supporting is no problem, as there is lots of GRP stanchions bolt the
crossbeams to.

Does any of you have experiences about such a project?
I have considered ½" marine plyvood, with 2 layers of mat on each side.
Or maybe a ½" sandwich plate (GRP-PU foam-GRP) well supported underneath.
That's lightweight and surprisingly strong.

Now, for the joining of the deck to the hull - should I make a solid joint,
joining deckplates and hull with mat? Or go for a bolt-and-flexible-sealant
type joint?

I want to have as much free deck space as possible.
The boat will be heavily ballasted, so the weight of the deck is not the
biggest issue.

NATO4EVER
Ole