Also get in touch with a carpenter that does installs of countertops. They use a very similar material and often have scrap pieces left over.
When they lay doen a sink, they cut out the round part for the sink, that's how much they got laying about. Silestone is something you could use.
True phenolic spacer material is expensive as it is epoxy impregnated layers of wood.
I fabbed a very thick gasket from an old not often used asbestos gasket material and it has been working great beneath my carb. Real thick stuff, about 3/16" or so. Just be careful of the dust.
Good luck,
Steve-O