As a simple solution to hold the superstructure down for towing try this.
if you source a long nut and bolt that will reach from a convenient point on a flat part of the superstructure, to just below deck level. Affix a suitable piece of square section wood dowel across under the deck, with the nut secured to the under side of the cross bar, then you can drop the bolt down through the superstructure and merely tighten with your fingers, this will be sufficient to hold it all down and prevent it from pulling off under a tow. Dotn try ightening the bolt down with a spanner, just incase you need to access inside the hull quickly in an emergency.
You can always hide the head of the bolt with a small cabinet or locker, so it does not stand out too much.
or if you cant find a bolt long enough, then buy a piece of threaded rod and cut a length to suit your requirements.
best of luck with the build, its looking great.