Moving on with the tugboat, I need to cover the big hole in the back, ideally with something reasonably flush to the deck, but as waterproof as possible. This rules out any combing around the edge.
In the end, I've used a couple of layers of plastic to form a sort of seal. It's not going to be very waterproof, even if I run some Vassalene around the edges, but hopefully, most of the water will stay the right side of the hull.
Along the back edge, a tongue locates under the deck. The other end is held down with high strength magnets. I can pull it up with finger pressure, especially once the towing post is fitted, but it seems to stay put pretty well, and may even be fine when hauling a light barge in the future.