Carry your brompton in a Birdy rucksack, then fold the bag back into a rucksack to return.
Buy a folding trailer from a DIY store (make sure it has soft tyres not hard plastic wheels!)
use luggage straps or zip ties to attach it to the rack, if you have one.
There are threads on here where I show pics of my using this method towing with my Strida, in order to pick someone up from a station.
Then strap their bags onto the trailer!
BTW Use a luggage strap to keep the trailer from folding when you go over a bump!