I carried my Brompton with one of these for a while:
http://idmybag.com/images/IDmyBag%20Stap%20Front.JPG
Cheap from many luggage stores.
Eventually I saw someone else leading their Brom along using the handlebars (which they hadn't folded down) and gave up on carrying it except for stairs.
A skatewheel conversion (google it) helped too.