Double plywood or double fiberglass for stiffness?

[LesForgue]

I might want to add stiffness to the bottom panels of my canoe.
I'm reluctant to add ribbing because I'm likely to kneel
at just about any random spot at some time or another.
So aside from the rib option,
would extra plywood for the entire bottom panels
OR
extra fiberglass cloth be the best for adding stiffness? Me I'm leaning towards the fiberglass cloth.
Thanks for helpful replies.

[jem]

If the boat is already assembled, fiberglass would be the easier way to go.

[LesForgue]

Thanks Matt!