Plywood for mould making

I am making a mould for a Moth and I am wondering what thicness plywood to use to go across the bulkheads to make the shape. I have tryied using 3mm ply but it was warping inbtween the bulkheads and not taking the shape of the hull properly. The bulkheads are spaced from 50 - 10cm apart and there is a fair bit of curve in the hull. Any suggestions for wood thicness would be greatly apreciated.
Thanks

 

Strip plank the mould with some cheap pine screwed to the bulkheads. Whatever thickness is cheapest. Countersink the screws, then you can fair without hitting them, then use bondo for the final finish.