I did not go through the plywood was I felt that it was too close to the edge of the fiddle. The 5/16 bungs would have been into the roundover on the bottom edge.

As Paul said drilling and tapping the Corian is not difficult. I started by drilling tiny holes through the fiddle into the Corian. Then drill with the right size drill for the tap.

#8 or #10 screws are plenty strong. I used siliconized latex caulk to keep the water out.