On Tue, 7 Sep 2004 00:01:52 +1000, "David Flew"
wrote:
Original construction looks like 15 mm ply ( probably marine back then - it
wasn't gold plated .. ) with fibreglass on top, and many many layers of
paint.

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I know from experience that 1/2 inch (12 mm) plywood is fairly bendy
and would not be a problem over the large radius you describe. Since
15 mm is only slightly thicker it would probably be OK also. If you
do a test and decide that too much force is required, you could make
shallow kerf cuts in the bottom of the panel with a table saw, and
then fill them later with thickened epoxy.