Need advice on mosaic project.......

I want to do a mosaic on the top of my patio table. Not on the glass. Want to remove glass & have SOMETHING cut to replace it, that I can do mosaic on. WHAT can I use that will withstand rain, damp, etc. Someone suggested marine plywood. I was thinking maybe that white composite plastic-like stuff that cutting boards are made of. Maybe 3/4 inch thick. Think that would work? Any other better ideas?

 

I don't see why it wouldn't it will be covered just be sure and seal your grout a couple of times