Before all that : http://www.surfersteve.com/smwavedesign.htm,

next question is - are you building an 1 off or a mold???

Anyway if i was doing it:

1. be 100% sure about design and final shape then make it from hard foam

2. When happy with the above , make a wood mold, 2 parts to be clamped

3. Then experiment clamping different stuff and playing with materials inside that shape untill i balance rigidity, weight and price

Thats it