Mast base flange. How to fit deck curvature?

[Chotu]

I am finishing up the design of my mast base right now.

It is not very easy or accurate to have the fabricator build a curvature into the mast base itself.

Instead, I would like to use some kind of deck plate below the mast base to account for the slight curvature. It’s maybe 1/4 of an inch of curvature. Similar to installing a deck hatch.

My question is, what material should I use for this deck plate?

Wood seems easy, but of course that is the worst thing you could use since it will rot. not a good place to have things rotting away. I have been using PVC plastic for deck hatches, but, I think that is too weak. It is kind of soft. It will probably crush under the forces of the rigging.