Hardwood planks for stiffness and strength and cedar to minimize weight. Use hardwood where it needs to be stiff and strong, at the leading edge. Two or three planks all the way to the deepest point then alternate with cedar to get the best compromise between strength and maximum weight.
Have a look at some Wayfarer CB pictures to get the idea. There are some on display here. And you may also get some good ideas from Philip Milanes. Unfortunately since Phil started selling his foils through chandlers his web-site went awful, you may have to follow some links to get to pictures of his foils.