Got one of these, recently.. http://carryfreedom.com/city.html as a Brompton Companion.
It takes a Waterproof Portage pack as cargo bag also, so stuff stays dry in there ,
more so than with the zip closed included bag ..
One can wear the trailer, then If needed on your back, with the Portage pack's straps .

Have had a Burly Flat bed, for a while, the Co-Op made ones , 2 20" QR wheels just the right load area

for my Roland AC-60 amp in it's padded Gig Bag, and Electric Mandolin's, Calton FG case..