Stewart,
I think I used 1.5mm hoop pine ply.
I made it by having a cedar backbone 33x19 tapers into provide the leading edge. A spacer of 1.5mm ply and cedar inside and a cedar trailing edge. Just form the shape around the spacer and made a carbon tube which I routed into the back end to take the sail bolt rope. Just cut a slot with diamond saw. I even had a tricky little sealed grove in the trailing edge of cedar so the halyard come down the inside of the mast and still keeping it sealed.

The mast I made was only 6m long, Carbon inside and out, cost $300AUD. I weighed it and did deflection tests on it and compared it to a 6m long superwing. about the same. But never sailed with it as I made it for a friend.

I'd have used 1mm if I had it.