There is something called 'bending' plywood that can mold nicely to the inside curve of your canoe and is about 1/8 thick(but expensive). I have used 'pattern plywood' (about 3/32'' thick and very light and cheap) which can be made to take on a nice curve to match the inside curve of the canoe; with a thin layer of glass and epoxy on the outside and around the corners, a fillet of epoxy and thickener on the inside, it is strong, durable and light. I have added a frame of 1/2 x 3/4 ash around the top of the box, and a top of 1/2 inch ply. peter