Some time back I discovered the Winthers Donky http://www.winther-bikes.com/pages_d...y_detaljer.php
On a bikeride in Denmark a year or two ago I accedently stumbeled upon one in a bike shop. We left it there thinking we could buy one later that day or the next day when arriving in Copenhagen. Mistake, we could not find the type of bike shop that would carry one with the limited time and energy we had left after the trip. Seing one lady walking with one did not help. Next time seing one was on the net. I think I followed a link in a Brompton forum/list and found descriptions of the trailer and the use of it by a happy owner.
EDIT: Here it is http://www.daclarke.org/AltTrans/donkey.html
Trying to find a shop that would sell me one was not possible, they would not ship it to me.
Then suddenly I spotted one on a local "craiglist", a person 1hr+ drive away from where I live offered one at approx £25,-. Did I buy it? YES. Brought it home today. it is not light and not really foldable so I can understand that they did not want to ship it.
Sellers pix: W Donky1.jpgW Donky2.jpgW Donky3.jpg
The tarp/cower and the mesh "door" is missing on mine but I can fix both.
I really like that it is both a hand cart and a trailer. Easy to walk and works fine when taking it on the bus since it is sturdy.
Somebody who can bend and weld can make one.