As I have just sent to Kirk, I use 3/8 ply with 2 by 3 exterior framing and a 17 foot kayak comes in around 150 lbs max - usually less. Kayak sits in cradle in box and is held down with more top cradles. To keep cost down on shipping boats or kits, I deliver to the depot and customer usually picks up at a depot at other end. If it is going to a biz, then truckiing company will deliver to the door, but if residential, cost is much higher, especially on a large item like a kayak or a long kit in a box or tube.