Reply by Bill White

Unless you want the bench to become a work of art, you could use a doubled up top from waterproof MDF. I have used Extira (brand). This stuff is heavy, tough, and waterproof. Can be machined, drilled, finished with shellac, and FLAT.
I have no connection with this product. Just a user.
It is dark brown, but who cares if it works. Look it up if ya wish.
Bill

-- bill@magraphics.us