I used plywood as a temporary centerboard refit when I repainted zzZippety Doh Dah. I believe it was 1/2 inch plywood that we cut to shape and glued/screwed together. We then covered it in several layers of epoxy and glassed it.
There is a pic of it right after being epoxied in this post about 3/4 down
Ben Lobaugh Online » ZDD: Haul out for refit and painting
Unfortunately I did not get a chance to snap a pic after it was glassed and before it was painted.
FYI - Ben Lobaugh Online » ZDD: Lucky sailing this summer