If you're after a wood finished look do as Big dutch said and scarf some ply together with epoxy. If you want to reduce weight and keep stiffness over the span. Lay up a layer of 400g cloth on the rear and bond it to some plascore honeycomb. 18/20mm and either lay glass up on the inside or repeat the outer skin inside.
If its going to be painted just lay glass up on each side of the panel. You could build a mould to include your camber and face it with laminate/formica. This will reduce 99% of your finishing work.
Lots of ways to do this depending on the finish you require.