Collapsible trailers?

Goudy

There are collapsible trailers, like Foldy, but they are designed to perhaps take up less space in a garage rather than lug up and down some stairs or through a house.

They stay rougly the same size but the axles and hitch fold up flat so you can store them against a garage wall.

You can buy motorbike trailers that are collapsible but they are really disassemblable rather than collapsible.
Perhaps contact Dave Cooper Trailers, see if they know a way of making a box trailer in a similar way.

Another option, if you're handy or know someone handy, would be convert a trailer.
I bought an old Jetski trailer and coverted it to a single off road motorcyle trailer.
It was really just the hitch and axle with nothing beyond the axle that stood up on it's end out of the way.
Worked really well.