He's going the distance .... he's going for speed .
Since you are using poly in the tray are going to do a little reenforcement in the tray area / bond line. I don't think much weight would be added if you use carbon and use 2 pound foam in the top gunwale area . Calculate the area you will be reinforcing , pick a cloth weight and figure the same for the resin. That should help you make a decision . 1 pound of cloth = 1 pound of resin and so forth . Look at Spruce aircraft supply , They have a spray polystyrene kit . I have no info on it , but I will.