Okay, the 145litre Really Useful Box is pretty much the ideal size.
Here's the Carry Freedom Y Large trailer:
Carry Freedom supply a set of rubber feet for mounting arbitrary boxes. Simply drill holes in the base of the box so that they line up with the holes in the trailer bed, preventing sideways or fore/aft movement. I've trimmed the exposed bolt heads on the inside of the box and covered them with Sugru to prevent damage to the contents:
RYB comes with a lid, which fits securely and is clamped in place by the handles at either end. A Wilko light-duty ratchet strap is less hassle than the Carry Freedom supplied velcro for securing the trailer:
Lightly hacked BS trailer reflectors and Smart rear light bracket complete the job:
This is the 'Strong white' box material, and it's the type of plastic that deforms and goes white with stress but doesn't shatter. It was easy to drill. The single ratchet strap is more that sufficient to hold the box securely in place, thanks to the feet. You can happily pick the whole thing up by the box and shake it.
My measurements suggest that there should be room inside the box for the dismantled trailer, though I'm going to wait until the Sugru has cured before trying it. Externally, the box is 620mm wide, so should fit through doorways with ease, even though the trailer won't.
I'm quite pleased with the result. This should be more than adequate for random shopping type stuff, and a large load space should make transportation of loose triffid cuttings to the tip a lot less hassle than trying to cram them into the naff plastic bags the council expect us to use for green waste.