T&T: Backing material

[Hal Anderson]

I previously installed a windlass on my boat and used the large washers
provided with the unit.  Under warranty I am about to replace the original
windlass with a new one.  I would like to provide a backing plate between
the deck and the washers/nuts to strengthen the installation.  What is the
best material for such a plate?
Thanks for any advice.

Hal
M/V Delilah