It never really occurred to me to only use a single reflector. I considered making triangles that would fit properly with the red tape, but I already had the reflectors and only one strip of red reflective tape left over from barakta's flag. I also considered hacking the box, but I decided that compromising the structural ridges would be silly.
Anyway, two reflectors makes it look wide, which can only be a good thing.
While we're on this thread, I used the trailer with the box for the first time the other day, to do a bulk shop from a Tesco about 4 fairly non-flat miles away. I had about 30kg of stuff in it, including 24 litres of orange juice and a couple of bulk packs of loo roll, and the box was only about half full. Hills were climbable with that load, though I would have wanted the trike for anything heavier. Descending was fine, but sharp braking and travelling at less than 5mph were a bit 'interesting'.
It's impressively non-rattly when there's a decent load in it. Even on the potholes. And drivers suddenly become uber-patient and polite.
What I did learn, was that there's no way I can lift the box off (or indeed onto) the trailer with that sort of weight in it, which means it won't go through the door. Not really a problem, just something to be aware of when loading up.