" I am in Australia, was hoping to bung it in a box trailer.
I might in the end go the other way and get a bigger one on a
trailer,but that would mean having three trailers to register each year. "

I have a 4'x 8' box type aluminum utility trailer also.
I was thinking of buying a 'sit on' kayak to play with at the beach.
If I do, I'll build a cradle to support the kayak on the utility trailer.
You might be able to design/build something similar.