Use the highest quality cabinet grade plywood you can get. It has 9 to 12 layers, and more rigidity to it's thickness. A 1/2" piece of cabinet grade is way stiffer than a 3-5 ply construction. grade.
As far as the structural pieces, I use Poplar. It's a clean, straight wood. Easy to work with and soaks up polyesters and epoxies well.
Remember, it's the glass work that determines the final strength. Mat for bulk, woven cloths for strength.