Hi Lionel, this is the way my superstructure is secured for towing on my Canning.......
2 small blocks of wood are fixed to the top of the front coaming, underneath the superstructure there is a lip at the bottom and the lip slides under the 2 blocks on the coaming. The superstructure is then placed front first over the coaming and slid back into place over the rear coaming. At the rear the superstructure is secured with 2 small brass pins that go through the superstructure and into the coaming.
You must make sure the coaming is very well secured and re-enforced and it wil tow without any fear of lifting the superstructure.
I have towed 2 blokes in a rowing boat around Llanberis last year with no problems.
The pics might make it clearer......
Mark.