With 'variety' I meant the different axle threadings and sizes, often worryingly close to one another.

The nuts of course just look like zinc-plated nuts, single chamfered, they grip the axle from the first quarter turn. UK-made from a supplier of fasteners for classic motorbikes. If I'd used these from the get-go I probably could have avoided faffing around removing spacers and the like.

My Carry Freedom is now equipped with a 60l Eurobox with lid and the supplied box quick release, a neat system as the entire load is secured with basically one strap. A second 30l Eurobox fits snug between the larger box and its lid too - so 90l total at 50 cm height if needed.
Euroboxes with the 60 x 40 cm footprint are inexpensive and available in folding/lightweight versions too.